220 COMPARATIVE ANATOMY. 



glance. Thus the dental formula of those animals the teeth of which are 

 represented in Figs. 177 to 181, would be 



Fig. 177. Hedgehog, i. f , c. g, pm. f , m. f . 



178. Dog, i. |, c. i, pm. f, m. f. 



179. Porcupine, i. }, c. g, m. f. 



180. Sheep, i. , c. ^, pm. , m. f. 



181. Catarrhine Monkey, i. f, c. }, pm. f, m. f. 



The variations in the dentition of the different groups of Mammals are so 

 exceedingly numerous that it is impossible to describe them in detail here, 

 and only the following points will be briefly remarked upon. 1 



The essential arrangement of the teeth is such that there is an alternation 

 between those of the upper and lower jaw ; thus the teeth of one jaw do not 

 correspond in position with those of the other, but with the interspaces 

 between them. 



A consideration of the rudimentary, functionless teeth which are commonly 

 present renders it probable that in the course of genealogical development the 

 teeth have undergone a decrease in number. 2 An increase in number, on the 

 other hand, must be always considered as an atavism. 



Finally, attention must be called to the commonly-existing sexual differ- 

 ences in dentition, as, for instance, in the Wild Boar, in the Narwhal 

 (Monodon),in the Dugong (Halicore), and in the Musk-deer. In the males 

 of these animals a modification of certain of the teeth (usually the canines) to 

 form tusks occurs, and these serve as fighting weapons. In the Elephant and 

 Walrus tusks are present in both sexes : in the former they correspond to 

 incisors, and in the latter to canines. 



Glands of the Mouth. 



The glands of the mouth, like those of the orbit and integument, 

 appear first in terrestrial animals, that is, from Amphibians on- 

 wards. They have the function of keeping moist the mucous 

 membrane which comes into contact with the outer air. From 

 being a,t first almost entirely unspecialised, and giving rise simply 

 to a slimy fluid, they become differentiated later into structures the 

 secretions of which take on a very important relation to digestion ; 

 they may also, as in the case of poisonous Snakes and Lizards, 

 constitute dangerous weapons of offence. 



With their gradually increasing physiological importance a 

 greater morphological complication both in number and arrange- 

 ment takes place. Their histological character also becomes 

 changed in such a manner that the three ordinary forms of glands, 

 i.e. tubular, compound-tubular, and acinous, may be 

 recognised. 



In the lower Vertebrata the two first forms preponderate, and 



1 These variations not only consist in the various positions and forms of the 

 teeth, but also in the typical arrangement of the enamel on the crowns. Thus in the 

 non-Ruminant Bunodontia the molars are tubercular, while in the Ruminant 

 Selenodontia the surface of the grinders is made up of crescent-shaped elevations. 



2 The last molar of Man, or so-called "wisdom-tooth," seems to be gradually dis- 

 appearing. It appears last and is lost first, and often does not reach the grinding 

 surface. 



