468 



MANUAL OF ELEMENTARY ZOOLOGY 



up, and mixed with saliva. When it is swallowed again it passes along a 

 muscular groove on the upper side of the second division of the stomach, 

 known as the reticulum or honeycomb-bag from the pattern on its 

 mucous membrane, into the psalterium or manyplies. The folded 

 walls of this chamber, covered with papillae, serve as a filter, through 

 which the food passes to the abomasum or reed, where the gastric juice 

 is secreted. Paired outgrowths of the frontal bones are common in 

 ruminants. In cattle, antelopes, sheep, and goats they are per- 

 manent and capped with hardened epidermis, the structures thus formed 

 being known as horns. In deer they lose their skin and form purely 

 bony antlers, which are shed yearly. 



B D E 



Fig. 346. — A transverse 

 section of an upper 

 molar tooth of a horse. 

 — From Theobald, 

 after Chauveau. 



A, External cement; B, ex- 

 ternal enamel ; C, dentine ; 

 D, internal enamel ; E, in- 

 ternal cement. 



Fig. 347. — A vertical section of 

 an incisor tooth of a horse. 

 — From Theobald, after 

 Chauveau. 



C, Cement ; E, enamel ; /, dentine. 



In the Perissodactyla the middle or third digit of each foot is larger 

 than the others and symmetrical in itself, and may be the only 

 complete digit. The premolars and molars are alike and have broad, 

 transversely ridged crowns. The stomach is simple, the caecum is large, 

 and there is no gall bladder. Horses, asses, and zebras belong to the 

 genus Equus. Here there is in each foot only one functional digit — the 

 third — with splints representing the metacarpals and metatarsals of 

 the second and fourth. The wrist of the horse is known as the " knee," 

 the ankle as the "hock." The metatarsals and metacarpals are the 

 "cannon bones," and the three phalanges of the single toe are the 

 "pastern," "coronet" or "little pastern," and "coffin bone" 

 respectively. The latter bears the hoof. The dental formula of the 



horse is i — lJa-3. The ridges on the grinding teeth are complicated, 



3. 1. 3. 3 

 some running along and some across the tooth, The enamel on the 



