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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 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.

1911 Britannica-Bird-Pelvis of a Fowl.png
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. Schultergürtel (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 Wirbelkörpergelenk 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,” Búll. 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 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 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.

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