372 



Comparative Anatomy — Its History, Aim, and Method 



There is some variation, however, in the number of segments (pha- 

 langes) in a finger or toe. Functional adaptation is effected by changing 

 proportions of segments and sometimes by the fusion of neighboring 

 bones (leg of bird; hindleg of frog: Fig. 295). In mammals the number 

 of vertebrae in the neck is nearly always seven. The exceptions are the 

 manatee (a sea cow) with six, and certain sloths having six, eight, or 

 nine. In birds the number of cervical vertebrae varies with the length 

 of the neck, but among mammals the number is seven in mouse and 

 man, in the giraffe, and also in the whale. In the latter, the seven verte- 

 brae are shortened into flat disks and more or less fused together. 



I 





f B \ 



Fig. 295. Adaptive modifications in the skeleton of the hindleg. (F) Femur; 

 (Fb) fibula; (M 1 , M\ M 5 ) metatarsals; (Ph) prehallux; (T) tibia; (Ts) tarsus. 

 Distal tarsals, solid black; proximal tarsals, stippled. 



The primitive hindleg (A) consists of three long segments — thigh, shank, and 

 foot. The frog's hindleg (B) acquires a fourth by elongation of proximal tarsals, 

 the tibiale and fibulare. In birds (E), fusion of elongated metatarsals produces a 

 segment whose length is further augmented proximally by addition of the distal 

 tarsals all fused together and solidly joined to the metatarsal complex. The proxi- 

 mal tarsals, likewise fused together, are solidly joined to the distal end of the tibia. 



The bird's ankle-joint, interposed between the tihio-tarsus and tarso-metatar- 

 sus, is a midtarsal joint and therefore does not correspond to the primitive ankle- 

 joint (indicated by arrows in A) which is between shank and tarsus. 



The bones of the reptilian tarsus (C, turtle; D, crocodilian) tend to segregate 

 into a proximal group and a distal group, within each of which occurs more or less 

 fusion — a condition suggestive of a midtarsal joint. 



The additional long segment in the leg of frog and bird is of mechanical advan- 

 tage in the leaping and hopping activities of these animals. 



