THE SKELETON. 71 



tebrates is similar to that of the carpus. Three bones, a 

 tibiale, intermedium, and fibula-re, compose the proximal 

 row, while five tarsalia form the distal row. The centrale 

 occupies the center of the tarsal region. In the adult mam- 

 mal, fusion of some of these elements occurs, so that not 

 more than seven distinct bones are present in any form. 

 The astragalus 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 

 metacarpus, which they resemble ( Fig. 43 ) . The first meta- 

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

 with the internal cuneiform and is also in contact with the 

 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 concavity for a sesamoid bone. These sesa- 

 moid bones are frequently 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. 35). 



The Equidae 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. 



