ENDOSKELETON AND VOLUNTARY MUSCLE 



71 



nine in number. The foot is provided with four or five digits 

 and seems to be derived from a pentadactyl type. Such a limb 

 is regarded as a fairly close approximation to the primitive type 

 of vertebrate limb from which the limbs of all vertebrates have 

 been derived. 



Departures from this primitive amphibian type of limb are 

 numerous in the higher vertebrates. Thus in the frog the radius 

 and ulna are fused and, likewise, the tibia and fibula. In other 

 animals, one component in the forearm or leg may be distinct 

 and the other greatly reduced. An extreme example of this 



Fig. 43. — Foot postures. A, plantigrade, bear; B, digitigrade, hyena; C, 

 unguligrade, horse. (A and B after Lull, Organic Evolution, C after Pander and 

 D'Alton.) 



among mammals is found in the horse, in which the fibula is 

 distinct but reduced in size to a thin rod. There is a reduction 

 of parts through loss as well as diminution in the more distal 

 elements of the limb of the horse. Thus metacarpal III is the 

 principal element of the metacarpus, metacarpals II and IV being 

 reduced to splints, while metacarpal V is missing. A similar 

 condition is found in the metatarsus. Of the digits only digit III 

 is present. The horse walks on the nail (hoof) of its middle 

 finger or toe. This type of foot posture is unguligrade (Fig. 43). 



Dogs and cats are digitigrade, i.e., they walk on their toes. 

 Strangely enough, the human limb has retained a number of 

 primitive features, such as distinct radius and ulna in the fore- 

 arm, and distinct tibia and fibula in the leg, though the fibula is 



