82 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



segment is directed ventrally, forming an angle with thigh and foot, and there 

 has also been some slight torsion. See further on these points hi Section III. 

 The thigh consists of one large bone, the femur, which fits into the acetabulum 

 by a prominent knob, the head of the femur. The shank is composed of tibia and 

 fibula, the latter being the smaller. The ankle is made up of five (or in some 

 turtles six) bones. At the bases of the tibia and fibula is a large, transversely 

 elongated bone, which is in reality composed of four bones fused (tibiale, 

 intermedium, centrale, fibulare). In some turtles the fibulare, at the base of the 

 fibula, is separate. Distal to this compound bone is a row of four bones, the 

 four tarsales, numbered from the preaxial (tibial) side to the postaxial (fibular) 

 side. The apparent fourth tarsale, the largest of the four, is really the fused 

 fourth and fifth tarsales. Distal to the tarsales are the five metatarsals, and 

 beyond these the digits, composed of bony joints or phalanges, terminating in 

 horny claws. The number of phalanges in the digits is that regarded as primi- 

 tive for vertebrates, namely, two phalanges to the first digit, and three to 

 all of the others (see Fig. 24). It is worthy of note that, in the turtles and 

 reptiles in general, the movement of the foot upon the leg occurs between 

 the two rows of tarsal bones, i.e., it is an intratarsal joint. Figures of the tarsal 

 bones of turtles are given in Wd, K, and R. 



5. The pelvic girdle and hind limb of birds. These structures, like the 

 remainder of the skeleton, are highly modified in birds, although consisting of 

 the same parts as in other vertebrates. Examine isolated backbones with the 

 pelvic girdles attached, or study the whole mounted specimens. The pelvic 

 girdle consists of three pairs of bones as in rep tiles, i.e., ilium, ischium, and pubis. 

 All three are fused on each side to form a continuous broad bone, the innominate 

 bone. The ilium is the largest and most dorsal part of the innominate bone. 

 It forms an elongated thin plate, concave in front, convex behind, extending from 

 the last thoracic vertebra to the tail region. It is fused along its entire length 

 with the synsacrum, the boundary between ilium and synsacrum generally 

 being marked by a suture. (In the bird embryo the ilium is articulated to 

 only two vertebrae which are the true sacral vertebrae.) The side of each 

 innominate bone is composed of the ischium, the only boundary mark between 

 this and the ilium being a large oval opening, the ilioischiac foramen. The pubis 

 is the long slender bone along the ventral border of the ischium, from which it is 

 separated by a more or less distinct suture, and the slitlike obturator foramen 

 which may be divided into two or more openings. Ilium, ischium, and pubis 

 contribute to the formation of the acetabulum. The anterior end of the pubis 

 is situated anterior to the acetabulum, the normal position of the pubis, but 

 during development the pubis turns posteriorly and comes to project beyond 

 the posterior end of the ischium. In the embryonic development of birds, 

 the three components of the innominate bone, ilium, ischium, and pubis, orig- 

 inate separately. Observe that neither pubic nor ischial symphyses are present, 



