THE ENDOSKELETON : GIRDLES, THE STERNUM, AND APPENDAGES 83 



but the two innominate bones are widely separated ventrally. This is probably 

 associated with the habit of laying large eggs. 



The hind limb offers several peculiarities. The femur has a large head 

 fitting into the acetabulum and a prominent projection lateral to the head, 

 called the great trochanter. The distal end of the femur is shaped like a pulley, 

 consisting of a central depression with curved ridges the condyles on either 

 side. Over the joint between thigh and shank is an extra small bone, the patella or 

 kneecap, not found in the lower vertebrates. The patella is a sesamoid bone, that 

 is, a bone developed in a tendon. Such sesamoid bones are quite common 

 in the limbs of higher vertebrates. The shank is composed of two bones, a 

 medial large one and a lateral short rudimentary bone. The large bone is the 

 tibiotarsus. It consists of the tibia fused at its distal end with the proximal 

 tarsal bones. The proximal end of the tibiotarsus has two condyles for articula- 

 tion with the condyles of the femur, and bears in front two diverging elevations 

 or crests for muscle attachments. The small bone of the shank is the fibula, 

 whose distal portion is atrophied. The distal end of the tibiotarsus has a 

 pulley-like surface for articulation with the succeeding bone, the raised articular 

 surfaces being named malleoli. Beyond the tibiotarsus is a long stout bone, 

 the tarsometatarsus, evidently formed by the fusion of three bones, as shown 

 by the three ridges on its distal end. The three fused bones are the meta- 

 tarsals (second, third, and fourth) ; in addition the tarsometatarsus includes 

 in its proximal portion the distal ankle bones. It will thus be seen that the ankle 

 bones do not exist separately in adult birds, but the proximal ankle bones are 

 fused to the lower end of the tibia, while the distal ones are fused to the upper 

 ends of the metatarsals. The ankle joint is, therefore, as in reptiles, an intratarsal 

 joint. The three metatarsals which are fused to form the tarsometatarsus are 

 the second, third, and fourth, but a remnant of the first metatarsal is present as 

 a small projection on the medial side of the distal end of this compound bone. 

 Each metatarsal articulates with its respective digit, composed of phalanges and 

 terminating in claws. Fifth metatarsal and digit are quite wanting. The gait 

 of birds is digitigrade. 



Draw the pelvic girdle and hind limb from the side. 



6. The pelvic girdle and hind limb of mammals. The pelvic girdle of 

 mammals is very similar to that of reptiles in shape, and similar to that of birds 

 in that the pelvic bones are fused into innominate bones. The girdle consists 

 of the usual three pairs of bones, pubis, ischium, and ilium, the three of each 

 side being indistinguishably fused into an innominate bone or hip bone. The 

 ilium is the most dorsal, most anterior, and largest of the three components of the 

 innominate bone. It articulates with the sacral vertebrae and terminates ante- 

 riorly and dorsally in a curved border, known as the crest of the ilium. The ilium 

 extends as far posteriorly as the acetabulum; the dorsal part of the girdle posterior 

 to the acetabulum is the ischium which is continuous with the ilium. The 



