314 VERTEBRATE AXIJIALS. 



formula for the portion of the jaw of the Chimpanzee figured above 

 (fig. 224) would be as follows : — 



I 2 ; c 1; pm 2 ; m 3. 



But this is only one half of the lower jaw, and the dental formula 

 must include both sides, so that it would be : — 



I 2— 2 ; cl— 1; pni 2— 2 ; «i 3— 3. 



That this would be the formula is at once evident, when it is 

 remembered that the two sides of the jaw of course contain exactly 

 the same teeth. Still, the formula as given above oidy includes the 

 lower jaw, and to render it jjerfect it must take in the teeth of the 

 upper jaw as well. This is effected by placing the figures in two 

 rows separated by short lines, all the figures in the upper ]-ow 

 referring to the upper jaw, and those in the lower row to the lower 

 jaw ; the shoi^t dashes between the figures of each row still indi- 

 cating the teeth on the two sides of the mouth. The complete 

 formula would therefore run as follows : — 



•2—2 1—1 2—2 3—3 



I - ; c ; pm ; m -- = .32. 



2—2 ' 1—1 ' ^ 2—2 ' 3—3 



In this way the dvidition — that is to say, the number and arrange- 

 ment of the teeth — can be presented in a manner which can be 

 instantly recognised by the uye. It nuist be remembered, however, 

 that the formula seldom exliibits the regularity of the one of the 

 Chimpanzee given above. The teeth are not necessarily the same 

 in both jaws, and in many (.-anes some may be altogether wanting. 

 To show this there ia subjoined the dental formula of a typical 

 Ruminant animal, such as a sheep : — 



.0-0 0-0 3—3 3—3 



I — ; c — ; pm ; m — = 32. 



3—3 1—1 ^ 3—3 3—3 



From this formula it will be seen that the sheep has 32 teeth in 

 both jaws taken together. The upper incisors and canines are 

 wanting, and there are three pi'semolars and three molars on each 

 side of the upjjer jaw. In the lowei' jaw there are six incisors, 

 two canines, and the same number of pnemolars and molars as in 

 the uppei' jaw. 



As regards the digestive system of Mammals, the alimentary 

 canal and digestive glands hAve on the whole the same general 

 structure and arrangement as in man (pp. 223, 224). Some very 

 remarkable modifications, however, in the structure of the stomach 

 and in the termination of the intestine, occur in certain Mammals ; 

 but these will be noticed in speaking of the orders in which they 

 occur. 



