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1911 Encyclopedia Britannica

Bird

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The common English name for feathered vertebrates, members of the class A y es. The word in Old Eng. is brid and in Mid. Eng. byrd or bryd, and in early uses meant the young or nestlings only. It is partly due to this early meaning that the derivation from the root of " brood " has been usually accepted; this the New English Dictionary regards as " inadmissible." The word does not occur in any other Teutonic language. As a generic name for the feathered vertebrates " bird " has replaced the older " fowl," a common Teutonic word, appearing in German as Vogel. " Bird," when it passed from its earliest meaning of " nestlings," seems to have been applied to the smaller, and " fowl " to the larger species, a distinction which was retained by Johnson. In modern usage " fowl," except in " wild-fowl " or " water-fowl," is confined to domestic poultry.

The scope of the anatomical part of the following article is a general account of the structure of birds (A y es ) in so far as they, as a class, differ from other vertebrates, notably reptiles and mammals, whilst features especially characteristic, peculiar or unique, have been dwelt upon at greater length so far as space permitted. References to original papers indicate further sources of information. For a comprehensive account the reader may be referred to Prof. M. Furbringer's enormous work Untersuchungen zur Morphologie and Systematik der Vogel, 4to., 2 vols. (1888); H. G. Bronn's Klassen and Ordnungen des Thierreichs, vol. vi., " A y es," Leipzig, completed 1893 by Gadow; and A. Newton's Dictionary of Birds, London, 1896. For the history of the classification of birds see the article Ornithology, where also the more important ornithological works are mentioned. EGG, Feather (including Moult), Migration, &c., also form separate articles to which reference should be made. In this article (A) the general anatomy of birds is discussed, (B) fossil birds, (c) the geographical distribution. of birds, (n) the latest classification of birds.

A. Anatomy Of Birds I. Skeleton. Skull. - When W. K. Parker wrote the account of the skull in the article Birds for the 9th edition of the Encyclopaedia Britannica, he had still to wrestle with the general problem of the composition and evolution of the skull. That chapter of comparative anatomy (together with other anatomical details, for which see the separate articles) is now dealt with in the article Skull; here only the most avine features are alluded to, and since some of Parker's original illustrations have been retained, the description has been shortened considerably.

One general feature of the adult bird's skull is the almost complete disappearance of the sutures between the bones of the cranium proper, whilst another is the great movability of the whole palatal and other suspensorial apparatus. The occipital condyle (fig. I) is a single knob, being formed almost wholly by the basi-occipital, while the lateral occipitals (often perversely called exoccipitals) take but little share in it. Part of the membranous roof between the supra-occipital and parietal bones frequently remains unossified and presents in the macerated skull a pair of fontanelles. The squamosals form the posterior outer margin of the orbits and are frequently continued into two lateral downward processes across the temporal fossa. One of these, the processus orbitatis posterior, often combines with an outgrowth of the alisphenoid, and may be, e.g. in cockatoos, continued forwards to the lacrymal bone, so as to form a complete infraorbital bridge. The posterior, so-called processus Zygomaticus is very variable; in many Galli it encloses a foramen by distally joining the orbital process. The ethmoid frequently appears on the dorsal surface between the frontals. There are three periotic bones (pro-, epi-, opisth-otic). The proOtic encloses between it and the lateral occipital the fenestra ovalis, into which fits the columella of the ear. The epiotic is often small, ossifies irregularly, and fuses with the supra-occipital. The opisthotic lies between the epiotic and the lateral occipital with which it ultimately fuses; in some birds, e.g. in Larus, it extends far enough to help to bound the foramen magnum. The basisphenoids are ventrally overlaid, and FIG. 1. - End view of skull of a Chicken fo three weeks old, X 8 diameters. Here the opisthotic bone appears in the occipital region, as in the adult Chelonian. (After W. K. Parker.) bo, Basi-occipital.

bt, Basi-temporal.

eo, Opisthotic.

f, Frontal.

fm, Foramen magnum. fo, Fontanella.

oc, Occipital condyle. op, Opisthotic.

p, Parietal.

pf, Post-frontal.

sc, Sinus canal in supra-occipital. ' b? 7.o o.c.' so, Supra-occipital.

sq, Squamosal.

8, Exit of vagus nerve.

later on fused with, a pair of membrane bones, the basi-temporals, homologous in part with the parasphenoid of lower vertebrates. They contribute to the formation of the auditory meatus, and of the right and left carotid canals which accompany the eustachian tubes.

In many birds the basisphenoids send out a pair of basipterygoid processes by which they articulate with the pterygoids. Dorsolaterally the basisphenoid is joined by the alisphenoid, which forms most of the posterior wall of the orbit. The orbito-sphenoids diverge only posteriorly, otherwise they are practically unpaired and form the median interorbital septum, which is very large in correlation with the extraordinary size of the eyeballs.

Prefrontal bones are absent; post-frontals are possibly indicated by a frequently occurring separate centre of ossification in the postorbital process, to which the frontals always contribute. The lacrymal is always present, and perforated by a glandular duct. Attached to it or the neighbouring frontal is often a supraorbital; infraorbitals occur also, attached to the jugal or downward process of the lacrymal. The nasals were used by A. H. Garrod to distinguish the birds as holorhinal (fig. 2) where the anterior margin of the nasal is concave, and schizorhinal where this posterior border of the outer nares is continued backwards into a slit which extends beyond the frontal processes of the premaxilla. Many birds possess a more or less well developed cross-joint in front of the frontals and lacrymals, perhaps best developed in Anseres and Psittaci. Owing to this joint the whole upper beak can be moved up and down with extra facility, according to the shoving forwards or backwards of the palato-pterygo-quadrate apparatus which moves sledge - like upon the cranial basis. The premaxilla is always unpaired, but each half has three long processes directed backwards; one fuses with the maxillary bone, another helps to form the anterior part of the palate, while the third, together with its fellow, forms the " culmen " and extends backwards to the frontals, or rather to the ethmoid which there crops up on the surface. The maxillaries (fig. 3) have besides others, a maxillo-palatine process directed inwards in a transverse horizontal direction. The palatines are long, always fused anteriorly with the premaxilla, and fre quently with the maxillo-palatine processes; posteriorly they slide upon the presphenoidal rostrum, and articulate in most birds with the pterygoids; they form the greater part of the palatal roof and border the choanae or inner nares. Between these, resting vertically upon the rostrum, appears the vomer; very variable in shape and size, often reduced to a mere trace, as in the Galli, or even absent, broken up into a pair of tiny splints in Pici.

The taxonomic importance of the configurations of the palate was first pointed out by J. de Cornay. T. H. Huxley, in 1868, divided the carinate birds into Dromaeo-, Schizo-, Desmo-, and Aegithognathae, an arrangement which for many years had a considerable influence upon classification. However, subsequent additions and corrections have detracted much from its value, especially when it became understood that the above sub-orders are by no means natural groups. Dromaeognathae have a struthious palate, with a broad vomer meeting in front the broad maxillo-palatal plates, while behind it reaches the pterygoids. The only representatives are the Tinamous. Schizognathae, e.g. fowls (fig. 4), pigeons, gulls, plovers, rails and penguins, have the vomer pointed in front while the maxillo-palatines are free, leaving a fissure between the vomer and themselves. The schizognathous formation is doubtless the most primitive, and its representatives form a tolerably natural FIG. 2. - Ripe Chick's head, 14 in. long; lower view X3 diameters. (After W. K. Parker.) as, Alisphenoid. Post-frontal.

bo, Basi-occipital. Pterygoid.

bt, Basi-temporal. Prenasal cartilage.

dpx, Dentary process of prePalatine process of pre maxilla. maxillary.

eo, Opisthotic. prp, Pterygoid process of sphe eu, Eustachian tube. noid.

f, Frontal. qj, Quadratojugal.

fin, Foramen magnum. so, Supra-occipital.

j, Jugal. sq, Squamosal.

1, Lacrymal. ty, Tympanic cavity.

mx, Maxilla. v, Vomer.

mxp, Maxillo-palatine process. 8, Exit of vagus nerve.

oc, Occipital condyle. 9, Exit of hypoglossal nerve. pa, Palatine.

e .o.

FIG. 3. - Skull of anold Fowl, X 12 diameter, upper view. (After W. K. Parker.)eo, Lateral occipital. npx, Nasal process of

eth,Ethmoid.f, Frontal. j, Jugal. 1, Lacrymal.n, Nostril. np, Upper process of nasal.

premaxillary.p, Parietal. pf, Post-frontal. px, Premaxilla. qj, Quadratojugal. so, Supra-occipital. sq, Squamosal.

pf, pg, pn, ppx, assembly. Desmognathae (fig. 5) were supposed to have the maxillopalatines united across the middle line, either directly or by the inter FIG. 4. - Skull of adult Fowl. This skull is unusually schizognathous, the vomer (v.) being very small, and the maxillo - palatine process (mxp) much aborted.

bo, Basi-occipital. bt, Basi-temporal. eo, Lateral occi pital.

eu, Eustachian tube.

ic, Internalcarotid. j, Jugal.

1, Lacrymal.

mx, Maxilla.

mxp, Maxillo-pala tine process. oc, Occipital con dyle.

pa, Palatine.

pf, Post-frontal. pg, Pterygoid.

prp, Pterygoid process of sphenoid. px, Premaxilla.

q, Quadrate.

qj, Quadratojugal. rbs, Rostrum of basi-sphenoid. so, Supra-occipital. v, Vomer.

8, Exit of vagus nerve.

9, Exit of hypo glossal nerve. (After W. K.

Parker.) mediation of ossifications in the nasal septum. This is a hopeless assembly. Parker and Ftirbringer have demonstrated that desmo FIG. 5. - Skull of nestling Sparrow !' hawk (Accipiternisus ), palatal view, X 2 diameters. The circular space on each side of the basi-temporal (bt.) is the opening of the anterior tympanic recess. The basi-pterygoids (bpg ) are mere knobs, and the common eustachian opening is seen between them. The maxillopalatine plates (mxp) are dotted to show their spongy character. bt, Basi-temporal.

bpg, Basi-pterygoid. eo, Lateral occipital. f, Frontal.

fm, Foramen magnum. Y j, Jugal.

1, Lacrymal.

"

u ?? mpg,Mesopterygoid process of W. K.

P tO u:..,.. ? `c Parker.

n - '8' mx, Maxillary. eo mxp, Maxillo-palatine 9 iid= _ process.

op, Opisthotic. pa, Palatine. so pg, Pterygoid.

sn, Nasal septum. 8, Exit of vagus so, Supra-occipital. nerve.

ty, Tympanic cavity. 9, Exit of hypo v, Vomer. glossal nerve. (After W. K. Parker.) gnathism has been produced in half a dozen ways, implying numerous cases of convergence without any nearer relationship than that they are all derived from some schizognathous group or other. The Aegithognathae, meant to comprise the passeres, woodpeckers and swifts, &c., are really schizognathous but with a vomer which is broadly truncated in front.

The remainder of the appendicular skeleton (fig. 6) of the head requires little description. The maxillaries are connected with the distal anterior corner of the quadrate by the thin, splint-like jugal and quadratojugal. The quadrate is invariably a conspicuous bone and movably articulating with the cranium and by a special process with the pterygoid. The mandible is composed of several bones as in reptiles. The os articulare bears on its inner side the inner mandibular process which serves for the insertion of part of the digastric muscle or opener of the mouth; another portion of this muscle is attached to the os angulare, which frequently forms a FIG. 6. - Skull of adult Fowl. Here the temporal fossa is bridged over by the junction of the post-frontal and squamosal processes (pf., sq.). The processes of the mandible (iap, pap ) are characteristic of this type, and of the anseres.

pa, Palatine.

pap,Posterior angular process of mandible.

pe, Ethmoid.

pf, Post-frontal.

pg, Pterygoid.

ps, Pre-sphenoid.

of px, Premaxilla.

q, Quadrate.

qj, Quadratojugal.

sa, Supra-angular or coronoid. so, Supra-occipital.

sq, Squamosal.

ty, Tympanic cavity.

v, Vomer.

1, Exit of olfactory nerve.

posterior mandibular process. The greater part of the under-jaw is formed by the right and left dentaries, which in all recent birds are fused together in front. Supra-angular and coronoid splint-bones serve for the insertion of part of the temporal or masseter muscle. Additional splints rest on the inner side of the jaw. Like the crocodiles, birds possess a siphonium, i.e. a membranous, or ossified, tube which rises from a pneumatic foramen in the os articulare, on the median side of the articulation, and passes upwards between the quadrate and lateral occipital bone, opening into the cavity of the middle ear.

The Hyoid apparatus is, in its detail, subject to many variations in accord with the very diverse uses to which the tongue of birds is III. 31 a, Angular of mandible. ar, Articular.

bt, Basi-temporal.

d, Dentary.

eo, Lateral occipital. eth, Ethmoid.

f, Frontal.

iap, Interangular process mandible.

ios, Interorbital septum. Jugal.

j, Lacrymal.

mx, Maxillary.

n, Nasal.

os, Orbito-sphenoid.

p, Parietal.

Px, Premaxilla. pto, ProOtic.

q, Quadrate.

qj, Quadratojugal.

put. It consists of (1) the basihyal variously called copula, or corpus linguae, or unpaired middle portion. (2) The urohyal likewise unpaired, rested ventrally on the larynx. (3) The os entoglossum originally paired, but coalescing into an arrow-headed piece, attached to the anterior end of the basihyal and lodged in the tongue proper. It is homologous with the distal ends of the ceratohyals or ventral elements of the hyoidean or second visceral arch. The dorsal or hyomandibular portion of this same arch is transformed into the auditory chain, ending in the fenestra ovalis.

(4) A pair of thyrohyals, homologous with the posterior hyoid horns of mammals, i.e. third visceral or first branchial arch. As the most developed pair in birds they are com monly, although wrongly, called the hyoid horns. They articulate upon facets of the hinder outer corners of the basihyal.

The vertebrae are stereospondylous, the centrum or body and the arch being com pletely fused into one mass, leaving not even a neuro-central suture. The arch alone sends 66r out processes, viz. the spinous process, the anterior and posterior oblique (commonly called preand post-zygapophyses), and the transverse processes. The latter articulate with the tuberculum of the corresponding rib, while the capitulum articulates by a knob on the side of the anterior end of the centrum. In the cervical region the ribs are much reduced, fused with their verte brae and enclosing the transverse canal or foramen. When the vertebrae are free their 6.h, centra articulate with each other by complicated joints, exhibiting four types. (I) Amphicoelous; each end of the centrum is concave; this, the lowest condition, is embryonic, but was retained in Archaeopteryx and in the thoracic vertebrae of Ichthyornis. (2) Procoelous, concave in front; only in the atlas, for the reception of the occipital condyle. (3) Opisthocoelous, or concave behind, only occasionally found in the thoracic region, e.g. Sphenisci. (4) Heterocoelous (fig. 8) or saddle-shaped; the anterior surface is concave in a transverse, but convex in a vertical direction, which on posterior surface shows the conditions reversed. This is the most perfect arrangement attained by the vertebral column, and is typical of, and restricted to, birds. The intervertebral joints are further complicated by the interposition of a cartilaginous or fibrous pad or ring. This pad varies much; it is morphologically the homologue of the pair of basiventral elements which by their lateral extension give origin to the corresponding ribs. Later those pads fuse with the anterior end of the centrum of the vertebra to which they belong; where the vertebral column is rendered inflexible, the disks are ossified with the centra and all trace of them is lost. Sometimes the pad is reduced to a ventral semi-ring or meniscus; it retains its largest almost original shape and size in the second vertebra, the axis or epistropheus, where it forms a separately ossifying piece which connects, and coossifies with, the odontoid process (the centrum of the atlas) and the centrum of the second vertebra. Sometimes the ventral portions of these pads form paired or un paired little ossifications, then generally described as intercentra; such are not uncommon on the tail. The atlas is composed of three pieces; a pair of lateral ele ptz pt.z W " ments (the right and left dorsal arch pieces) joining above the spinal cord, and a ventral piece equivalent to the first basiventral elements, i.e. serially homologous with the intervertebral pads. In the adults the atlas forms a more or less solid ring. A remnant of the chorda dorsalis and its sheath persists as the ligamentum suspensorium between the central portions of the successive vertebrae.

In birds we distinguish between the following regions of the axial skeleton. (I) Cervical vertebrae, or those between the skull and the first vertebra which is connected with the sternum by a pair of complete ribs. The last I to 5 of these vertebrae have movable ribs which do not reach the sternum, and are called cervico-dorsals. (2) Dorsals, those which begin with the first thoracic rib, and end at the last that is not fused with the ilium. The term " lumbar " vertebrae is inapplicable to birds. (3) Pelvic, all those which are fused with the iliac portion of the pelvis, generally a considerable number. (4) Caudal, those which are not connected with the pelvis. It is to be noted that often no absolute line of demarcation can be drawn in regard to these regions, their definitions being rather convenient than morphological.

In comparison with all other vertebrates the number of neck-vertebrae of the birds is considerably increased; the lowest number, 14 to 15, is that of most Passeres and many other Coraciomorphae; the largest numbers, 20 or 21, are found in the ostrich, 23 in Cygnus olor and 25 in the black swan. Dorsal vertebrae frequently have a ventral outgrowth of the centrum; these hypapophyses may be simple vertical blades, I-shaped, or paired knobs; they serve for the attachment of the thoracic origin of the longus collianticus muscle, reaching their greatest development in Sphenisci and Colymbidae. In many birds some of the thoracic vertebrae are more or less coOssified, in most pigeons for instance the 15th to 17th; in most Galli the last cervical and the next three or four thoracics are coalesced, &c. The pelvic vertebrae include of course the sacrum. There are only two or three vertebrae which are equivalent to those of the reptiles; these true sacrals are situated in a level just behind the acetabulum; as a rule between these two primary sacral vertebrae issues the last of the spinal nerves which contributes to the composition of the sciadic plexus. These true sacrals alone are connected with the ilium by processes which are really equivalent to modified ribs; but the pelvis of birds extends considerably farther forwards and backwards, gradually coming into contact with other vertebrae, which in various ways send out connecting transverse processes or buttresses, and thus become preand post-sacral vertebrae (fig. 9). The most anterior part of the ilium often overlaps one or more short lumbar ribs and fuses with them, or even a long, complete thoracic rib. Similarly during the growth of the bird the posterior end of the ilium connects itself with the transverse processes of vertebrae which were originally free, thus transforming them from caudals into secondary post-sacrals. Individual, specific and generic variations are frequent.

The last six or seven caudal vertebrae coalesce into the pygostyle, an upright blade which carries the rectrices. Such a pygostyle is absent in Archaeopteryx, Hesperornis, Tinami and Ratitae, but it occurs individually in old specimens of the ostrich and the kiwi. In Ichthyornis it is very small. In all the Neornithes the total number of caudal vertebrae, inclusive of those which coalesce, is reduced to at least 13.

Sternum (figs. 10 and 11). - Characteristic features of the sternum are the following. There is a well-marked processus lateralis anterior (the right and left together equivalent to the mammalian manubrium), which is the product of two or three ribs, the dorsal parts of which reduced ribs remain as cervico-dorsal ribs. Then follows the rib-bearing portion and then the processus lateralis posterior; this also is the product of ribs, consequently the right and left processes together are equivalent to the xiphoid process or xiphisternum of the mammals. The lateral process in most birds sends out an outgrowth, directed out and upwards, overlapping some of the ribs, the processus obliquus. The median and posterior extension of the body of the sternum is a direct outgrowth of the latter, therefore FIG. 7. - Os hyoides of adult Fowl, X II diameters.

c.h, Ceratohyals (confluent).

b.h, The so-called basihyal, answering to the first basibranchial of a fish.

b.br, Basi-branchial, or urohyal, answering to the rest of the basibranchial series.

c.br, e.br, together form the thyrohyal, answering to the first ceratoand epibranchials.

FIG. 8. - A cervical vertebra from the middle of the neck of a Fowl; natural size. a, Side view; b, upper view; c, lower view; pr.z, pre-zygapophyses; pt.z, post-zygapophyses.

FIG. 9. - The " sacrum" of a young Fowl; natural size, seen from below. d.l, Dorso-lumbar, s, sacral, c, caudal vertebrae.

FIG. 10. - A side view of the Chick's sternum.

called meta-sternum. The anterior margin of the sternum, between the right and left anterior lateral processes receives in sockets the feet of the coracoids. Between them arises a median crest, which varies much in extent and composition, and is of considerable taxonomic value. It is represented either by a spina interna or by a spina externa, or by both, or they join to form a spina communis which is often very large and sometimes ends in a bifurcation. Eventually, when the right and left feet of the coracoids overlap each other, the anterior sternal spine contains a foramen. The keel, or carina sterni, is formed as a direct cartilaginous outgrowth of the body of the sternum, ossifying from a special centre. This keel is much reduced in the New. Zealand parrot, Stringops, less in various flightless rails, in the dodo and solitaire. It is absent in the Ratitae, which from this feature have received their name, but considerable traces of a cartilaginous keel occur in the embryo of the ostrich, showing undeniably that the absence of a keel in the recent bird is not a primitive, fundamental feature. The keel has been lost, and is being lost, at various epochs and by various groups of birds. The swimming Hesperornis (see Odontornithes) was also devoid of such a structure. In many birds the spaces between the metasternum and the posterior processes and again the spaces between this and the oblique process are filled up by proceeding ossification and either remain as notches, or as fenestrae, or they are completely abolished so that the breastbone is turned into one solid more or less oblong plate.

Shoulder Girdle. - Scapula, coracoid and clavicle, meet to form the foramen triosseum, through which passes the tendon of the supracoracoideus, or subclavius muscle to the tuberculum superius of the humerus. The coracoid is one of the most characteristic bones of the bird's skeleton. Its upper end forms the acrocoracoid process, against the inner surface of which leans the proximal portion of the clavicle. From the inner side of the neck of the coracoid arises the precoracoidal process, the remnant of the precoracoid. Only in the ostrich this element is almost typically complete, although soon fused at either end with the coracoid. Near the base of the precoracoidal process is a small foramen for the passage of the nervus supracoracoideus. In most birds the feet of the coracoids do not touch each other; in some groups they meet, in others one overlaps the other, the right lying ventrally upon the left. The scapula is sabre-shaped, and extends backwards over the ribs, lying almost parallel to the vertebral column. This is a peculiar character of all birds. The clavicles, when united, as usual, form the furcula; mostly the distal median portion is drawn out into a hypocleidium of various shape. Often it reaches the keel of the sternum, with subsequent syndesmosis or even synostosis, e.g. in the gannet. In birds of various groups the clavicles are more or less degenerated, the reduction beginning at the distal end. This condition occurs in the Ratitae as well as in the well-flying Platyrcecinae amongst parrots.

The fore-limb or wing (fig. 12); highly specialized for flight, which, initiated and made possible mainly by the strong development of quill-feathers, has turned the wing into a unique organ. The humerus with its crests, ridges and processes, presents so many modifications characteristic of the various groups of birds, that its configuration alone is not only of considerable taxonomic value but that almost any genus, excepting, of course, those of Passeres, can be " spotted " by a close examination and comparison of this bone. When the wing is folded the long glenoid surface of the head of the humerus is bordered above by the tuberculum externum or superius, in the middle and below by the tuberculum medium or inferius for the insertion of the coraco-brachialis posterior muscle. From the outer tuberculum extends the large crista superior (insertion of pectoralis major and of deltoideus major muscles). The ventral portion of the neck is formed by the strong crista inferior, on the median side of which is the deep fosses subtrochanterica by which air sacs enter the humerus. On the outer side of the humerus between the head and the crista inferior is a groove lodging one of the coraco-humeral ligaments. The distal end of the humerus ends in a trochlea, with a larger knob for the ulna and a smaller oval knob for the radius. Above this knob is often present an ectepicondylar process whence arise the tendons of the ulnar and radial flexors. The radius is the straighter and more slender of the two forearm bones. Its proximal end forms a shallow cup for articulation with the outer condyle of the humerus; the distal end bears a knob which fits into the radial carpal. The ulna is curved and rather stout; it articulates with both carpal bones; the cubital quills often cause rugosities on its dorsal surface. Of wrist-bones only two remain in the adult bird; the original distal carpals coalesce with the proximal end of the metacarpals. These are reduced, in all birds, to three, but traces of the fourth have been observed in embryos. The first metacarpal is short and fuses throughout its length with the second. This and the third are much longer and fuse together at their upper and distal ends, leaving as a rule a space between the shafts. The pollex and the third finger are as a rule reduced to one phalanx each, while the index still has two. The first and second fingers frequently carry a little claw. The greatest reduction of the hand-skeleton is met with in Dromaeus and in A pteryx, which retain only the index finger. It is of importance for our understanding of the position of the Ratitae in the system, that the wing-skeleton of the ostrich and rhea is an exact repetition of that of typical flying birds; the bones are much more slender, and the muscles are considerably reduced in strength also to a lesser extent in numbers, but the total length of the wing of an ostrich or a rhea is actually and comparatively enormous. Starting with the kiwi and cassowary, people have got into the habit of confounding flightless with wingless conditions. It is absolutely certain that the wings of the Ratitae bear the strongest testimony that they are the descendants of typical flying birds.

FIG. 12. - Bones of Fowl's right wing, adult, nat. size.

h, Humerus.

r, Radius.

u, Ulnar.

r', u', Radial and ulnar carpal bones; with the three digits I., II., III.

FIG. I I. - Sternum of a Chick (Gallus domesticus ) three days old, lower view, X three diameters. The cartilage is shaded and dotted, and the bony centres are light and striated.

The pelvis (fig. 13), consisting of the sacrum (already described) and the pelvic arch, namely ilium, ischium and pubis, it follows that only birds and mammals possess a pelvis proper, whilst such is entirely absent in the Amphibia and in reptiles with the exception of some of the Dinosaurs. The ventral inner margin of the preacetabular portion of the ilium is attached to the pre-sacral vertebrae, whilst the inner and dorsal margin of the postacetabular portion is attached to the primary sacral and the postsacral vertebrae. In rare cases the right and left preacetabular blades fuse with each other above the spinous processes. In front of the acetabulum a thick process of the ilium descends to meet the pubis, and a similar process behind meets the ischium. The acetabulum is completely surrounded by these three bones, but its cup always retains an open foramen; from its posterior rim arises the strong antitrochanter. The ischium and postacetabular ilium originally enclose the ischiadic notch or incisura ischiadica. This primitive condition occurs only in the Odontornithes, Ratitae and Tinami; in all others this notch becomes converted into a foramen ischiadicum, through which pass the big stems of the ischiadic nerves and most of the bloodvessels of the hind-limb. The pubis consists of a short anterior portion ( spina pubica or pectineai process, homologous with the prepubic process of Dinosaurs) and the long and slender pubis proper (equivalent to the processus lateralis pubis of most reptiles). The shaft of the pubis runs parallel with that of the ischium, with which it is connected by a short ligamentous or bony bridge; this cuts off from the long incisura pubo-ischiadica a proximal portion, the foramen obturatum, for the passage of the obturator nerve. Only in the ostrich the distal ends of the pubes meet, forming a daggershaped symphysis, which is curved forwards. The pectineal process is variable; it may grow entirely from the pubis, or both pubis and ilium partake of its formation, or lastly its pubic portion may be lost and the process is entirely formed by the ilium. It is largest in the Galli and some of the Cuculi, in others it is hardly indicated. It served originally for the origin of the ambiens muscle (see Muscular System below); shifting or disappearance of this muscle, of course, influences the process.

1 The Hind Limb

2 pigeon

3 Ear

4 Nose

5 Venous System

6 Lymphatic System

7 Authorities

8 Nearctic Subregion

The Hind Limb

The femur often possesses a well visible pneumatic foramen on the median side of the proximal end of its shaft. The inner condyle, the intercondylar sulcus, and a portion only of its outer condyle, articulate with corresponding facets of the tibia. The outer condyle articulates mainly with the fibula. There is a patella, intercalated in the tendon of the femori-tibialis or extensor cruris muscle. In Colymbus the patella is reduced to a small ossicle, its function being taken by the greatly developed pyramidal processus tibialis anterior; in Podiceps and Hesperornis the patella itself is large and pyramidal. The distal half of the fibula is very slender and normally does not reach the ankle-joint; it is attached to the peroneal ridge of the tibia. On the anterior side of the tibia, is the intercondylar sulcus, which is crossed by an oblique bridge of tendon or bone, acting as a pulley for the tendon of the extensor digitorum communis muscle. The condyles of the tibia are in reality not parts of this bone, but are the three proximal tarsalia which fuse together and with the distal end of the tibia. The distal tarsalia likewise fuse together, and then on to the upper ends of the metatarsals; the tarsale centrale remains sometimes as a separate osseous nodule, buried in the inter-articular pad. Consequently the ankle-joint of birds is absolutely cruro-tarsal and tarso-metatarsal, i.e. intertarsal, an arrangement absolutely diagnostic of birds if it did not also occur in some of the Dinosaurs. Of the metatarsals the fifth occurs as an embryonic vestige near the joint; the first is reduced to its distal portion, and is, with the hallux, shoved on to the inner and posterior side of the foot, at least in the majority of birds. The three middle metatarsals become fused together into a cannon bone; the upper part of the third middle metatarsal projects behind and forms the so-called hypotarsus, which in various ways, characteristic of the different groups of birds (with one or more sulci, grooved or perforated), acts as guiding pulley to the tendons of the flexor muscles of the toes. Normally the four toes have two, three, four and five phalanges respectively, but in Cypselus the number is reduced to three in the front toes. Reduction of the number of toes (the fifth shows no traces whatever, not even in Archaeopteryx) begins with the hallux, which is completely or partly absent in many birds; the second toe is absent in Struthio only. The short feet of the penguins are quite plantigrade, in adaptation to which habit the metatarsals lie in one plane and are incompletely co-ossified, thus presenting a pseudo-primitive condition.

Literature. - Only a mere fraction of the enormous literature dealing with the skeleton of birds can here be mentioned.

M. E. Alix, Essai sur l'appareil locomoteur des oiseaux (Paris, 1874); E. Blanchard, " Recherches sur les caracteres osteologiques des oiseaux appliques a la classification," Ann. Sci. Nat. Ser. iv., t. xi.; W. Dames, " Uber Brustbein Schulterand Beckengi.irtel der Archaeopteryx," Math. Naturw. Mitch., Berlin, vii., 1897, pp. 47649 2; T. C. Eyton, Osteologia avium (London, 1858-1881), with many plates; C. Gegenbaur, Untersuch. z. vergl. Anat. d. Wirbelthiere, I. Carpus and Tarsus, II. Schultergurtel (Leipzig, 1864-1865); P. Harting, L'A ppareil episternal des oiseaux (Utrecht, 1864); T. H. Huxley, " On the Classification of Birds and on the Taxonomic Value of the Modifications of certain of the Cranial Bones..." P.Z.S., 1867; G. Jaeger, " Das Wirbelkorpergelenk der Vogel," Sitzb. K. Ak. Wiss., Wien, xxxiii., 1858; A. Johnson, " On the Development of the Pelvic Girdle and Skeleton of the Hind-limb in the Chick," Q.J.M.S., xxiii., 1883, pp. 399-4 11; K. F. Kessler, " Osteologie der Vogelftisse," Bull. Soc. Imp. Nat., Moscow, xiv., 1841; B. Lindsay, " On' the Avian Sternum," P.Z.S., 1885; E. Mehnert, " Entwickelung des Ospelvis der Vogel," Morph. Jahrb., xiii., 1877; A. B. Meyer, Abbildungen von Vogel-Skeletten (Dresden, 1879); St G. Mivart, " On the Axial Skeleton of the Ostrich, Struthionidae, Pelecanidae," Trans. Zool. Soc. viii., 1874; x., 1877; E. S. Morse, " On the Carpus and Tarsus of Birds," Ann. Lyc. N.H., 'New York, x., 1874; J. S. Parker, " Observations on the Anatomy and Development of Apteryx," Phil. Trans., 1890, pp. I-110, 17 pls.; W. K. Parker, numerous papers in Trans. L.S., R.S. and Z.S., e.g. " Osteology of Gallinaceous Birds," T.Z.S., v., 1863; " Rhinochetus," ibid. vi.; "Skull of Aegithognathous Birds," ibid. x., 1878; " Skull in the Ostrich Tribe," Phil. Trans. vol. 156, 1866; " Skull of Common Fowl," ibid. vol. 159, 1870; " Skull of Picidae," T. Linn. Soc., 1875; " Monograph on the Structure and Development of the Shoulder-girdle and Sternum," Ray Soc. London, 1868; W. P. Pycraft, " On the Morphology and Phylogeny of the Palaeognathae ( Ratitae and Crypturi ) and Neognathae," Trans. Zool. Soc. xv., 1900, pp. 149-290, pls. 42-45; id. " Some points in the morphology of the Palate of the Neognathae," T. Linn. Soc. 28, pp. 343-357, pls. 31-32; P. Suschkin, " Zur Morphologie des Vogelskelets. I. Schadel von Tinnunculus," Mem. Soc., Moscow, xvi., 1900, pp. 1 -63, pls.

2. Muscular System. Of the muscles of the stem or axis, those of the neck and tail are well-developed and specialized, while those of the lower back are more or less reduced, or even completely degenerated owing to the rigidity of this region, brought about by the great antero-posterior extent of the pelvis.

The muscles of the limbs show a great amount of specialization, away from the fundamental reptilian and mammalian conditions. The muscles of the fore limbs are most aberrant, but at the same time more uniformly developed than those of the hinder extremities. The reasons are obvious. The whole wing is a unique modification, deeply affecting the skeletal, muscular and tegumentary structures, but fluttering, skimming, sailing, soaring are motions much more akin to one another than climbing and grasping, running, scratching, paddling and wading.

The modifications of the hind-limbs are in fact many times greater (such as extremely long legs, with four, three or only two toes; very short legs, almost incapable of walking, with all four toes directed forwards, or two or one backwards, and two or more connected and therefore bound to act together, in various FIG. 13. - Pelvis and caudal vertebrae of adult Fowl, side view, natural size. Il. Ilium; Is, ischium; Pb, pubis; d.l, dorso-lumbar vertebrae; Cd, caudal vertebrae; Am, acetabulum.

ways. Thus it has come to pass that the muscles of the hind limbs are, like their framework, more easily compared with those of reptiles and mammals than are the wings, whilst within the class of birds they show an enormous amount of variation in direct correlation with their manifold requirements. The only really aberrant modifications of the wing-muscles are found in the Ratitae, where they are, however, all easily explained by reduction, and in the penguins, where the wings are greatly specialized into blades for rowing with screw-like motions.

The wing of the bird is folded in a unique way, namely, the radius parallel with the humerus, and the whole wrist and hand with their ulnar side against the ulna; upper and forearm in a state of supination, the hand in that of strong abduction. Dorsal and ventral bending, even in the extended wing, is almost impossible. Consequently only a few of the original extensor muscles have been preserved, but these are much modified into very independent organs, notably the extensor metacarpi radialis longus, the ext. metac. ulnaris and the two radio- and ulnari-metacarpi muscles, all of which are inserted upon the metacarpus by means of long tendons. The chief muscular mass, arising from the sternum in the shape of a U, is the pectoralis muscle; its fibres converge into a strong tendon, which is inserted upon the greater tubercle and upper crest of the humerus, which it depresses and slightly rotates forwards during the downstroke. This great muscle covers completely the supracoracoideus, generally described as the second pectoral, or subclavius muscle, in reality homologous with the mammalian supraspinatus muscle. This arises mostly from the angle formed by the keel with the body of the sternum, passes by a strong tendon through the foramen triosseum, and is inserted upon the upper tubercle of the humeral crest, which it rotates and abducts. The extent of the origin of this muscle from the sternum, on which it leaves converging, parallel or diverging impressions, is of some taxonomic value.

Much labour has been bestowed by A. H. Garrod and Max Farbringer upon the investigation of the variations of the inserting tendons of the patagial muscles (fig. 14), mainly from a taxonomic FIG. 54. - Wing muscles of a Goose. Bi, Biceps; Elast. sec., elastic vinculum and Exp.sec., expansor secundariorum; Pt.br and Pt.lg, short and long propatagial muscles; Tri, triceps.

point of view. The propatagialis longus muscle is composed of slips from the deltoid, pectoral, biceps and cucullaris muscles. Its strong belly originates near the shoulder joint from clavicle, coracoid and scapula. Its elastic tendon runs directly to the carpus, forming thereby the outer margin of the anterior patagium, or fold of skin between the upper and forearm, which it serves to extend, together with the propatagialis brevis muscle. This runs down the anterior and outer side of the upper arm, and is attached to the proximal tendon of the extensor metacarpi radialis longus, a little below the outer condyle of the humerus. In most birds the tendon is split into several portions, one of which is often attached to the outer side of the ulna, below the elbow joint, while others are in variable but characteristic ways connected with similar slips of the propatagialis longus. The posterior patagium, the fold between trunk and inner surface of the upper arm, is stretched by the metapatagialis muscle, which is composed of slips from the serratus, superficialis, latissimus dorsi and the expansor secundariorum muscles. This, the stretcher of the cubital quills, is a very interesting muscle. Arising as a long tendon from the sterno-scapular ligament, it passes the axilla by means of a fibrous pulley, accompanies the axillary vessels and nerves along the humerus, and is inserted by a few fleshy fibres on the base of the last two or three cubital quills. Here, alone, at the distal portion of the tendon, occur muscular fibres, but these are unstriped, belonging to the category of cutaneous muscles. We have here the interesting fact that a muscle (portion of the triceps humeri of the reptiles) has been reduced to a tendon, which in a secondary way has become connected with cutaneous muscles, which, when strongly developed, represent its belly.

The flexor digitorum sublimis muscle arises fleshy from the long elastic band which extends from the inner humeral condyle along the ventral surface of the ulna to the ulnar carpal bone, over which the tendon runs to insert itself on the radial anterior side of the first phalanx of the second digit. Owing to the elasticity of the humerocarpal band the wing remains closed without any special muscular exertion, while, when the wing is extended, this band assists in keeping it taut. The arm-muscles have been studied in an absolutely exhaustive manner by Fiirbringer, who in his monumental work has tabulated and then scrutinized the chief characters of fourteen selected muscles. The results are as interesting from a morphological point of view (showing the subtle and gradual modifications of these organs in their various adaptations), as they are sparse in taxonomic value, far less satisfactory than are those of the hind-limb. He was, however, the first to show clearly that the Ratitae are the retrograde descendants of flying ancestors, that the various groups of surviving Ratitae are, as such, a polyphyletic group, and he has gone fully into the interesting question of the development and subsequent loss of the power of flight, a loss which has taken place not only in different orders of birds but also at various geological periods, and is still taking place. Very important are also the investigations which show how, for instance in such fundamentally different groups as petrels and gulls, similar bionomic conditions have produced step by step a marvellously close convergence, not only in general appearance, but even in many details of structure.

Of the muscles of the hind-limbs likewise only a few can be mentioned. The ambiens muscle, long and spindle-shaped, lying immediately beneath the skin, extending from the pectineal process or ilio-pubic spine to the knee, is the most median of the muscles of the thigh. When typically developed its long tendon passes the knee j oint, turning towards its outer side, and lastly, without being anywhere attached to the knee, it forms one of the heads of the flexor perforates digit, ii. or iii. One of the functions of this peculiar muscle (which is similarly developed in crocodiles, but absent, or not differentiated from the ilio-tibial and ilio-femoral mass, in other vertebrates) is that its contraction helps to close the second and third toes. Too much has been made of this feature since Sir R. Owen ( Cyclop. Anat. Phys. i. p. 296, 1835), following G. A. Borelli ( De motu animalium, Rome, 1680), explained that birds are enabled to grasp the twig on which they rest whilst sleeping, without having to make any muscular exertion, because the weight of the body bends the knee and ankle-joints, over both of which pass the tendons of this compound muscle. There are many perching birds, e.g. all the Passeres, which do not possess this muscle at all, whilst many of those which have it fully developed, e.g. Anseres, can hardly be said to " perch." Garrod went so far as to divide all the birds into Homalogonatae and Anomalogonatae, according to the presence or absence of the ambiens muscle. This resulted in a failure. To appreciate this, it is sufficient to enumerate the birds without the critical muscle: Passeriformes and Coraciiformes, without exception; Ardeae and Podiceps; lastly various genera of storks, pigeons, parrots, petrels and auks. The loss has taken place, and still takes place, independently in widely different groups. It follows, first, that the absence of this muscle does not always indicate relationship; secondly that we can derive birds that are without it from a group which still possess it, but not vice versa. The absence of the ambiens muscle in all owls, which apparently use their feet in the same way as the Accipitres (all of which possess it), indicates that owls are not developed from the latter, but from a group which, like the other Coraciiformes, had already lost their muscle.

Garrod further attributed much taxonomic value to the caudilio femoralis muscle (fig. 55). This, when fully developed, consists of two parts, but inserted by a single ribbon-like tendon upon the hinder surface of the femur, near the end of its first third; the caudal part, femoro-caudalis, expressed by Garrod by the symbol A, arises from transverse processes of the tail; the iliac part ( accessorofemoro-caudal of Garrod, with the symbol B ), arises mostly from the outer surface of the postacetabular ilium. Of course this doubleheaded condition is the more primitive, and as such exists in most nidifugous birds, but in many of these, as well as in many nidicolous birds, either the caudal or the iliac head is absent, and in a very few (Cancroma, Dicholophus, Steatornis and some Cathartes ) the whole muscle is absent. The caud-ilio flexorius (semitendinosus of most authors) arises from the transverse processes of the tail, and from the distal half of the postacetabular ilium, thence passing as a broad ribbon to the popliteal region, where it splits into two portions. One of these, broad and fleshy, is inserted upon the posterior surface of the distal third of the femur. This portion, morphologically the original, was named the " accessory semitendinosus " with the symbol Y; the other portion descends on the hinder aspect of the leg and joins the fascia of the inner femoral head of the gastrocnemius muscle. In many birds the insertion is shifted from the femur to the neck of the tibia, in which case the " accessory head " is said to be absent, a condition expressed by Garrod by the symbol X. By combining the four symbols A, B, X, Y, according to their presence or absence, Garrod got a considerable number of formulae, each of which was overruled, so to speak, by the two categories of the presence or absence of the ambiens muscle. It needs hardly to be pointed out why such a purely mechanical scheme was doomed to Jiletapatag. Tri. Exp.sec. From Newton's by permission of A. & C. Black.

failure. Its author, with a considerable mathematical and mechanical bias, reckoned entirely with the quantity, not with the quality of his units, and relied almost implicitly upon his formulae. It is, however, fair to state that his system was not built entirely upon these muscular variations, but rather upon a more laborious combination of anatomical characters, which were so selected that they presumably could not stand in direct correlation with each other, notably the oil-gland, caeca, carotids, nasal bones and above all, the muscles of the thigh. He was, indeed, the first to show clearly the relationship of the heron-like birds with the Steganopodes; of storklike birds with the American vultures; the great difference between the latter and the other birds of prey; the connexion of the gulls and auks with the plovers, and that of the sand-grouse with the From Newton's FIG. 15. - Left thigh-muscles of a Rail. Outer view after removal of the Il.fb, ilio-fibularis and Il.tib, ilio-tibialis.

A, Caudal.

B, Iliac portion of caud-ilio-femoralis.

X, Caud-ilio-flexorius.

Y, " Accessory " portion of the same. Pif, Pubischio-femoralis.

pigeon

discoveries expressed in the new terms of the orders Ciconiiformes and' Charadriiformes. These are instances, now well understood, that almost every organic system, even when studied by itself, may yield valuable indications as to the natural affinities of the various groups of birds. That Garrod has so very much advanced the classification of birds is ultimately due to his comprehensive anatomical knowledge and general insight.

To return to these thigh muscles. The most primitive combination, ambiens and A B X Y, is the most common; next follows that of A X Y, meaning the reduction of B, i.e. the iliac portion of the caud-ilio-femoralis; A B X and B X Y are less common; A X and X Y are rare and occur only in smaller groups, as in subfamilies or genera; B X occurs only in Podiceps. But the greatest reduction, with only A remaining, is characteristic of such a heterogeneous assembly as Accipitres, Cypselidae, Trochilidae, Striges and Fregata. This fact alone is sufficient proof that these conditions, or rather reductions, have been acquired independently of the various groups. A B Y, A Y, A B, X Y and B do not occur at all, some of them for obvious reasons, Occasionally there is an instructive progressive evolution expressed in these formula; for instance Phaethon, in various other respects the lowest of the Steganopodes, has A X Y, Sula and Phalacrocorax have A X, Fregata, the most specialized of these birds, has arrived at the reduced formula A. Further, the combinations B X Y and A X Y cannot be derived from each other, but both directly from A B X Y in two different directions. Keeping this in mind, we may fairly conclude that the flamingo with B X Y points to an ancestral condition A B X Y, which is still represented by Platalea and Ibis, whilst the other storks proper have taken a different line, leading to A X Y.

Literature. - Well nigh complete lists of the enormous myological literature are contained in Fiirbringer's Untersuchungen zur Morphologie and Systematik der Vogel, and in Gadow's vol. Vogel of Bronn's Klassen and Ordnungen des Tierreichs. Only a few papers and works can be mentioned here, with the remark that few authors have paid attention to the all-important innervation of the muscles.

A. Carlsson, Beitrdge zur Kenntniss der Anatomie der Schwimmvogel; K. Svensk, Vet. Ak. Handlinger. J. G. No. 3 (1884); A. Alix, Essai sur l'appareil locomoteur des oiseaux (Paris, 1874); H. Gadow, Zur vergl. Anat. der Muskulatur des Beckens and der hinteren Gliedmasse der Ratiten, 4° (Jena, 1880); A. H. Garrod, " On Certain Muscles of the Thigh of Birds and on their value in Classification," P.Z.S., 1873, pp. 624-644; 1874, pp. 111 -123. Other papers by Garrod, 1875, PP. 339.-348 (deep plantor tendons); 1876, pp. 506-519 (wing-muscles [[[Anatomy]] of Passeres), &c.; J. G. de Man, Vergelijkende myologische en neurologische Studien over Amphibien en Vogels (Leiden, 1873), ( Corvidae); A. Milne-Edwards, Recherches anatomiques et paliontologiques pour servir a l'histoire des oiseaux fossiles de la France (Paris, 1867-1868), torn. i. pls. ix.-x. ( Aquila and Gallus); R. Owen, article "A y es," Todds' Cyclopaed. of Anat. and Phys. i. (London, 1835); " On the Anatomy of the Southern Apteryx," Trans. Zool. Soc., iii., 1849; A. Quennerstedt, " Studier i foglarnas anatomi," Lunds Univers. Aarsk., ix., 1872 (hind-limb of swimming birds); G. Rolleston, " On the Homologies of Certain Muscles connected with the Shoulder-joint," Trans. Linn. Soc., xxvi., 1868; R. W. Shufeldt, The Myology of the Raven (London, 1891); M. Watson, " Report on the Anatomy of the Spheniscidae," Challenger Reports, 1883.

3. Nervous System. Brain. - The more characteristic features of the bird's brain show clearly a further development of the reptilian type, not always terminal features in a direct line, but rather side-departures, sometimes even a secondary sinking to a lower level, and in almost every case in a direction away from those fundamentally reptilian lines which have led to the characters typical of, and peculiar to, the mammals.

The forebrain forms the bulk of the whole brain, but the large size of the hemispheres is due to the greater development of the basal and lateral portions (pedunculi cerebri and corpora striata ), while the pallium (the portion external to the lateral ventricles) is thin, and restricted to the median side of each hemisphere. As a direct result of this undoubtedly secondary reduction of the pallium - due to the excessive preponderance of the basal and lateral parts - the corpus callosum (i.e. the transverse commissure of the right and left pallium) is in birds reduced to a narrow flat bundle of a few white fibres; it is situated immediately above and behind the much stronger anterior commissure, i.e. the connexion between the corpora striata, or chief remaining part of the hemispheres. Owing to the small size of the olfactory lobes the anterior arms of the latter commissure are wanting. There is very little grey matter in the cortex of the hemispheres, the surface of which is devoid of convolutions, mostly quite smooth; in others, for instance pigeons, fowls and birds of prey, a very slight furrow might be compared with the Sylvian fissure.

The Thalamencephalon is much reduced. The epiphysis, or pineal body, is quite as degenerate as in mammals, although still forming a long stalk as in reptiles. In birds, this stalk consists entirely of blood-vessels, which in the adult enclose no terminal vesicle, and fuse with the membranous linings of the skull. The midbrain is represented chiefly by the optic lobes, the cortex of which alone is homologous with the corpora quadragemina of the mammals. Their transverse dorsal connexion is the posterior commissure; otherwise the whole roof portion of the midbrain is reduced to a thin membrane, continuous with that which covers the Sylvian aqueduct, and this ventricle sends a lateral cavity into each optic lobe, as is the case in reptiles. The right and left lobes themselves are rent asunder (so to speak), so that they are freely visible from above, filling the corners formed by the hemispheres and the cerebellum. The latter is, in comparison with mammals, represented by its middle portion only, the vermis; in a sagittal section it shows an extremely well developed arbor vitae, produced by the transverse, repeated folding of the whole organ. In comparison with reptiles the cerebellum of birds shows high development. Forwards it covers, and has driven asunder, the optic lobes; backwards it hides the much shortened medulla oblongata.

Several futile attempts have been made to draw conclusions as to the intelligence of various birds, from comparison of the weight of the whole brain with that of the body, or the weight of the hemispheres with that of other parts of the central nervous system.

The brachial plexus is formed by four or five of the lowest cervical nerves; the last nerve of this plexus often marks the boundary of the cervical and thoracic vertebrae. The composition of the plexus varies much, not only in different species, but even individually. The most careful observations are those by Fiirbringer. The serial number of these nerves depends chiefly upon the length of the neck, the extremes being represented by Cypselus (loth-14th cervical) and Cygnus (22nd-24th), the usual numbers of the common fowl being the 13th-17th nerves.

The Crural Plexus is divided into a crural, ischiadic and pubic portion. The first is generally composed of three nerves, the hindmost of which, the furcalis, issues in most birds between the last two lumbo-sacral vertebrae, and then divides, one half going to the crural, the other to the sciatic portions. The obturatorius nerve invariably comes from the two main stems of the crural. The ischiadic portion consists generally of five or six nerves, which leave the pelvis as one thick system through the ilio-ischiadic foramen. The last nerve which contributes to the ischiadic plexus leaves the spinal


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Bibliography Information
Chisholm, Hugh, General Editor. Entry for 'Bird'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/bri/b/bird.html. 1910.

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