THE SKELETON 8 I 



adult mammal, fusion of some of these elements occurs, so 

 that not more than seven distinct bones are present in any 

 form. The talus contains the tibiale and intermedium, while 

 the fourth and fifth tarsalia form the cuboid. 



The metatarsus consists of five bones, the first of which 

 is rudimentary, while the other four are longer than the meta- 

 carpus, which they resemble (Fig. 44). The first metatarsal 

 is smaller than any bone of the tarsus. It articulates with 

 the internal cuneiform and is also in contact with second 

 metatarsal. The four long metatarsals have a slight dorsal 

 convexity, each one consisting of a base or enlarged proximal 

 part, a shaft or middle portion, and a head or distal rounded 

 articular process. On the plantar surface of the head of each 

 metatarsal is a median spine, on each side of which is a con- 

 cavity for a sesamoid bone. These sesamoid bones are fre- 

 quently lost in cleaning the skeleton. 



The number of functional metatarsals in mammals varies 

 from one to five. Usually the number of metacarpals and 

 metatarsals is the same, but in the tapir there are four meta- 

 carpals and but three metatarsals. All Monotremata, Car- 

 nivora, and Primates have five metatarsals, but in the cats, 

 dogs, and hyenas the first metatarsal is vestigial. In early 

 geological . times all mammals possessed five metatarsals, the 

 number of which in many cases has gradually diminished 

 owing to environment, as is shown in the phylogeny of the 

 horse (Fig. 36). 



The Equidte possess but one functional metatarsal and two 

 rudimentary ones, although Orohippus, the ancestor of the 

 horse, had three functional metatarsals. 



The phalanges of the cat's foot are twelve in number. Each 

 metatarsal except the first supports three phalanges. The 

 proximal phalanges are the stoutest, and the distal the smallest. 

 Each distal phalanx bears a claw. 



In all five-toed animals, such as Primates, there are four- 

 teen phalanges, each digit being composed of three, except 

 the hallux, which has two. 



