| 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. |
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 syndosmosis 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.
| 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. |
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 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 Apteryx, 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.
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 blood-vessels 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 dagger-shaped 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.
| 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. |
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 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. Mitsh., Berlin, vii., 1897, pp. 476-492; T.C. Eyton, Osteologia avium (London, 1858-1881), with many plates; C. Gegenbaur, Untersuch. z. vergl. Anat. d. Wirbelthiere, I. Carpus und Tarsus, II. Schultergurtel (Leipzig, 1864-1865); P. Harting, L’Appareil épisternal 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 Vögel,” 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. 399-411; K.F. Kessler, “Osteologie der Vogelfüsse,” Bull. Soc. Imp. Nat., Moscow, xiv., 1841; B. Lindsay, “On the Avian Sternum,” P.Z.S., 1885; E. Mehnert, “Entwickelung des Ospelvis der Vögel,” Morph. Jahrb., xiii., 1877; A.B. Meyer, Abbildungen van Vögel-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 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, pis. 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.