INTEODUCTION 



13 



facet is, among the heavy ungulates (Amblypoda and Proboscidea), almost flat, 

 but as a rule there are two prominent ridges separated by a deep groove. 



The distal portion of the astragalus may be prolonged, making a neck, the 

 end either resting by a flat or slightly arched articular facet on the navicular, 

 or presenting a broad arched roller facet to the navicular, cuboid and cal- 

 caneum (Fig. 15, c). On the posterior face of the astragalus lie one or two 

 facets for the calcaneum, and one for the cuboid. In primitive forms there 

 occurs a foramen above the proximal facet. 



The calcaneum (Fig. 16) is an elongated angular bone, the rear of which 

 is prolonged in an abbreviated stem {tiiber calcis, tc), which in plantigrade 

 animals rests on the ground and makes ^ 



the heel, while in digitigrade types it 

 is raised and directed obliquely up- 

 ward and backward. Distally the 

 calcaneum comes in contact with the 

 cuboid, and anteriorly with the upper 

 part of the astragalus. A process 

 (sustentaculum) on the inner side 

 bounds the sustentacular facet [as) 

 for the astragalus ; while on the op- 

 posite outer side occurs the peroneal 

 facet {p); and occasionally there is also 

 a facet for the fibula {v). The cuboid is 

 an irregular four-sided bone, somewhat 

 higher than broad. The navicular 

 extends over the three adjacent cunei- 

 form bones. 



The metatarsals {m) correspond in 

 general with the metacarpals, though 

 their proximal articular ends are usually flattened and pressed close against 

 the tarsalia, the articulations being usually about on a level. But the fourth 

 and fifth {m.IV. and V.) metatarsals articulate with the cuboid, the others 

 each articulating with a single tarsal bone. By the development of m.III. and 

 IV. the same reductions may take place on the hind- as on the fore-foot, the 

 first or great toe (hallux) being the first lost, then the fifth, followed by 

 the second, and in the most extreme cases (horse) the fourth may also be 

 sacrificed. Among ruminants m.III. and IK fuse as in the fore -limb to 

 make the cannon bone. 



The phalanges and sesamoids of the hind-foot diff"er as a rule but slightly 

 in number, size and form from those of the front foot. 



Fig. 16. 



Left calcaneum of the dog (^-1) and of Macrauclienia 

 (B). tc, Tuber calcis ; as, sustentacular facet for astra- 

 galus ; p', outer or peroneal facet for astragalus ; cub 

 distal facet for cuboid ; p, facet for fibula. 



Mammalian Tarsus. 



Astragalus 



Calcaneum 



Navicular 



Cuboid. 



Entocuneiform 



Mesocuneiform 



Ectocuneiform 



Reptilian Tarsus. 



Intermedium. 

 Fibulare. 

 Centrale. 

 Tarsale IV. 

 Tarsale I. 

 Tarsale II. 

 Tarsale III. 



In general the development of the limb depends closely on the modifications 

 and habits of the animal. As in many cases the limbs are used exclusively for 



