184 ANATOMY OF VERTEBRATES. 



The femur in the Frog, fig. 44, 65, is a long slender bone, with 

 a slight double bend; the head is expanded, convex, and ter- 

 minal ; the back part of the upper fourth of the shaft shows a 

 longitudinal ridge ; the distal end is expanded and truncate. 

 Both ends of the femur are usually in the state of epiphyses. 

 The tibia and fibula are confluent longitudinally, but preserve 

 their respective medullary canals, and indicate their transcend- 

 ental distinction by an anterior and posterior longitudinal furrow 

 at the expanded ends of the seeming single bone ; usually, also, 

 by a perforation from before backward. A single epiphysis con- 

 stitutes each articular end. The astragalus a, and calcaneum cl, 

 are much elongated. The former is slightly bent. They com- 

 monly coalesce at their proximal and distal extremities ; at the 

 former, by means of an epiphysis ; at the latter, with the connate 

 representatives of the naviculare, s, and cuboides, b. Two cunei- 

 form bones remain distinct and support the three inner toes, i, it, 

 in : a third expanded bone projects, like a supplemental digit, ci, 

 from the inner (tibial) side of the tarsus ; it may represent the 

 * entocuneiform.' One (Rana) or two (Pipa) sesamoid bones are 

 developed in the extensor tendons behind the tibio-tarsal joint: 

 their function is that of the lever part of the calcaneum. The 

 first metatarsal supports two phalanges, I ; the second, two ; the 

 third, three ; the fourth, four ; and the fifth, three. 



In Bufo agua I found a semiossified tubercle upon the proximal 

 end of each ilium. In Pipa, the confluent calcaneum and cuboid 

 form a long three-sided bone with the angles sharp : the long- 

 astragalus presents a similar form. 



To the student of Comparative Anatomy entering upon the 

 vast domain of that science with ideas of the bones derived from 

 those of the human skeleton, and associating the special shapes 

 and proportions they there present with the names that have been 

 learnt from Anthropotomy, few parts are more perplexing or de- 

 ceptive than the pelvis in the Chelonian reptiles. Viewing, for 

 example, that of the Trionyx, as it is represented in fig. 116, he 

 would conclude h to be the iliac bones, and i the pubic bones, 

 separated at the symphysis ; or as answering to the parts so called 

 in the sino-le ' os innominatum ' of man. The rectification of the 

 error affords a valuable lesson of the unimportance of size and 

 shape in determining special homology, and of the necessity of 

 knowing the foetal as well as the adult conditions of the human 

 pelvis. He would learn, first, that the threefold nature of the 

 ' innominate ' bone, which is transitory in man, is permanent 

 in the reptile ; next, that the bone which is largest and 



