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ANATOMY]

BIRD

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

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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

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gnathism has been produced in half a dozen ways, implying numerous cases of convergence without any nearer relationship than that they III 16⭑

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

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 commonly, although wrongly, called the hyoid horns. They articulate upon facets of the hinder outer corners of the basihyal.

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The vertebrae are stereospondylous, the centrum or body and the arch being completely fused into one mass, leaving not even a neuro-central suture. The arch alone sends bar out processes, viz. the spinous process, the anterior and posterior oblique (commonly called pre- and 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 vertebrae and enclosing the transverse canal or foramen. When the vertebrae are free their centra articulate with each other by complicated joints, exhibiting four types. (1) 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 b.h, The so-called basi- attained by the vertebral column, and is hyal, answering typical of, and restricted to, birds. The interto the first basi- vertebral joints are further complicated by branchial of a fish. the interposition of a cartilaginous or fibrous b.br, Basi-branchial, pad or ring. This pad varies much; it is or urohyal, an- morphologically the homologue of the pair swering to the of basiventral elements which by their rest of the basi- lateral extension give origin to the correbranchial series. sponding ribs. Later those pads fuse with c.br, e.br, together the anterior end of the centrum of the form the thyro- vertebra to which they belong; where the hyal, answering vertebral column is rendered inflexible, the to the first cer- disks are ossified with the centra and all ato- and epi- trace of them is lost. Sometimes the pad is branchials. 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 coössifies 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

FIG. 7.-Os hyoides

of adult Fowl.

c.h, Ceratohyals (confluent).

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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 elements (the right and left dorsal arch pieces) FIG. 8.-A cervical vertebra from the joining above the middle of the neck of a Fowl; natural size. spinal cord, and a a, Side view; b, upper view; c, lower view; ventral piece equivalent to the first basipr.z, pre-zygapophyses; pt.s, post-zygapophyses. ventral 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 arial skeleton. (1) 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 1 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 a 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, 1-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 coössified, 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 FIG. 9.-The "sacrum" are connected with the ilium by pro- of a young Fowl; natural cesses which are really equivalent to size, seen from below. modified ribs; but the pelvis of birds d.Dorso-lumbar, s, sacral, extends considerably farther forwards, caudal vertebrae. and backwards, gradually coming into contact with other vertebrae, which in various ways send out connecting transverse processes or buttresses, and thus become pre- and 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, con- the Chick's sternum. FIG. 10.-A side view of sequently 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

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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 varics 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 Zealand New parrot, Stringops,

FIG. 11.-Sternum of a Chick (Gallus domesticus) three days old, lower view. The cartilage is shaded and dotted, and the bony centers are light and striated.

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 claviele, 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 fossa 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

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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 (q.v.), 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 pectineal 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

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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 caractères ostéologiques des
oiseaux appliquées à la classification," Ann. Sci. Nat. Ser. iv., t. xi.;
W. Dames, Über Brustbein Schulter- und Beckengürtel der
Archaeopteryx," Math. Naturw. Mitth., Berlin, vii., 1897, pp. 476-
492; T. C. Eyton, Osteologia avium (London, 1858-1881), with
many plates: C. Gegenbaur, Untersuch. 2. vergl. Anat. d. Wirbel-
there, Ca L'appareil épisternal des oiseaux (Utrecht, 1864): T. H
und Tarsus, II. Schultergürtel (Leipzig, 1864-1865);
P. Harting,
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 Wirbelkörpergelenk der Vögel," Sitsb. 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. 399-411; K. F. Kessler, "Osteologie der

Vogelfüsse," Bull. Soc. Imp. Net, Moscow, xiv., 1841; B. Lindsay, "On the Avian Sternum," P.ZS, 1885; E. Mehnert, "Entwickelung des Ospelvis der Vögel," 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. 1-110, 17 pls: W. K. Parker, numerous papers in Trass. 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," bid. 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. Schädel von Tinnunculus," Mem. Soc., Moscow, xvi, 1900, pp. 1-63, pls.

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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. 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.

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

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

ways. Thus it has come to pass that the muscles of the hind The flexor digitorum sublimis muscle arises fleshy from the long limbs are, like their framework, more easily compared with those elastic band which extends from the inner humeral condyle along of reptiles and mammals than are the wings, whilst within the the tendon runs to insert itself on the radial anterior side of the first the ventral surface of the ulna to the ulnar carpal bone, over which class of birds they show an enormous amount of variation in phalanx of the second digit. Owing to the elasticity of the humerodirect correlation with their manifold requirements. The only carpal band the wing remains closed without any special muscular really aberrant modifications of the wing-muscles are found in exertion, while, when the wing is extended, this band assists in keepthe Ratitae, where they are, however, all easily explained by exhaustive manner by Fürbringer, who in his monumental work ing it taut. The arm-muscles have been studied in an absolutely reduction, and in the penguins, where the wings are greatly has tabulated and then scrutinized the chief characters of fourteen specialized into blades for rowing with screw-like motions. selected muscles. The results are as interesting from a morphoThe wing of the bird is folded in a unique way, namely, the radius logical point of view (showing the subtle and gradual modifications of parallel with the humerus, and the whole wrist and hand with their these organs in their various adaptations), as they are sparse in ulnar side against the ulna; upper and forearm in a state of supinataxonomic value, far less satisfactory than are those of the hind-limb. He was, however, the first to show clearly that the Ratitae are the tion, the hand in that of strong abduction. Dorsal and ventral bending, even in the extended wing, is almost impossible. Conse of surviving Ratitae are, as such, a polyphyletic group, and he has retrograde descendants of flying ancestors, that the various groups quently only a few of the original extensor muscles have been pre-gone fully into the interesting question of the development and served, but these are much modified into very independent organs, subsequent loss of the power of flight, a loss which has taken place notably the extensor metacarpi radialis longus, the ext. metac. ulnaris and the two radio- and ulnari-metacarpi muscles, all of which are periods, and is still taking place. Very important are also the innot only in different orders of birds but also at various geological inserted upon the metacarpus by means of long tendons. The chief vestigations which show how, for instance in such fundamentally muscular mass, arising from the sternum in the shape of a U, is the different groups as petrels and gulls, similar bionomic conditious have pectoralis muscle; its hibres converge into a strong tendon, which is produced step by step a marvellously close convergence, not only inserted upon the greater tubercle and upper crest of the humerus, in general appearance, but even in many details of structure. which it depresses and slightly rotates forwards during the downstroke. This great muscle covers completely the supracoracoideus, tioned. The ambiens muscle, long and spindle-shaped, lying imOf the muscles of the hind-limbs likewise only a few can be mengenerally described as the second pectoral, or subclavius muscle, inmediately beneath the skin, extending from the pectineal process or reality homologous with the mammalian supraspinatus muscle. This arises mostly from the angle formed by the keel with the body of ilio-pubic spine to the knee, is the most median of the muscles of the the sternum, passes by a strong tendon through the foramen tri- joint, turning towards its outer side, and lastly, without being anythigh. When typically developed its long tendon passes the kneeosseum, and is inserted upon the upper tubercle of the humeral crest, which it rotates and abducts. The extent of the origin of this where attached to the knee, it forms one of the heads of the flexor muscle from the sternum, on which it leaves converging, parallel or perforatus digit, ii. or iii. One of the functions of this peculiar diverging impressions, is of some taxonomic value. muscle (which is similarly developed in crocodiles, but absent, or not Much labour has been bestowed by A. H. Garrod and Max Für- vertebrates) is that its contraction helps to close the second and differentiated from the ilio-tibial and ilio-femoral mass, in other bringer upon the investigation of the variations of the inserting third toes. Too much has been made of this feature since Sir R. tendons of the patagial muscles (fig. 14), mainly from a taxonomic 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."

Pt.lg.

Lig.

Elast, sec.

Propalag

Pl.br. Bi.

Tri.

Melapatag

Exp.sec.

From Newton's Didionary of Birds, by permission of A. & C. Black.

FIG. 14.-Wing muscles of a Goose. Bi, Biceps; Elast. sec., elastic vinculum and Exp.sec., expansor secundariorum; Pl.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 propalagialis 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.

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. 15). 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

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