440 ZOOLOGY FOR MEDICAL STUDENTS chap. 



"raised to form knobs (bunodont, e.g. Pig), or ridges (lophodont, e.g. 

 Rhinoceros), or crescentic elevations .(selenodont, e.g. Sheep). As 

 these projections are worn down by use the opposing surfaces of the 

 upper and lower grinders are kept irregular by the differing resistance 

 to wear afforded by the harder enamel and the softer dentine. Where 

 ■the grinder-teeth are exposed to particularly great wear and tear owing 

 to the harsh nature of the food (e.g. Horse, Elephant) the crown part 

 of the tooth may be greatly exaggerated in relative size, the projecting 

 ridges- being of great depth and in this case they are protected from the 

 danger of being broken off by being embedded in a matrix of bone 

 (cement). Such teeth with specially deep crowns are termed hypsodont 

 to distinguish them from ordinary short-crowned (brachydont) teeth. 



As a rule the mammalian tooth reaches a definitive size after which 

 it ceases to grow and its pulp cavity has its originally widely-open com- 

 munication with the underlying tissue constricted to a comparatively 

 small pore. Where, however, the tooth is subject to great wear and tear 

 it is usually only at a late period that this shutting off of the pulp cavity 

 takes place : it remains widely open for a prolonged period during which 

 the tooth continues to grow in length (incisors of Rodents such as the 

 Rabbit or Beaver or Rat). 



The alimentary canal shows well-marked differentiation into distinct 

 regions — buccal cavity, pharynx, oesophagus, stomach, and intestine. 

 The gill pouches present in the embryo soon disappear except the 

 first which remains as the Eustachian tube and the tympanic cavity. 

 There is a well-developed larynx functioning as the organ of voice. The 

 intra-pulmonary bronchus divides within the lung in a complicated tree- 

 like manner, the ultimate twigs ending in minute chambers termed 

 intundibula, the walls of which bulge out into alveoli lined with a rich 

 network of capillary blood-vessels. The air-channels in the lung of the 

 mammal thus end blindly as in other vertebrates with the exception of 

 birds. The stomach is usually a simple dilatation of the alimentary 

 canal but in some cases (Ruminants) it forms a series of chambers 

 specialized for different functions. The most conspicuous peculiarity of 

 the intestine is that its hinder portion (large intestine) is dilated and 

 greatly increased in length to form a reservoir in which the faecal material 

 accumulates. In all except the very lowest mammals the cloaca becomes 

 split into a ventral portion continuous with the urinogenital sinus and a 

 dorsal portion continuous with the rectum. The original cloacal opening 

 thus comes to be represented by two distinct openings — urinogenital and 

 anal — which are separated by a considerable space — the perineum. As 

 in other terrestrial vertebrates there are well-developed salivary glands 



