554 CHORDATA 



are symmetrical, only those of one side need be enumerated, and in case 

 that one kind be absent the deficiency is indicated by a zero. Thus the 

 dental formula of man is fl j|;of the reindeer, in which in the upper jaw 

 incisors and canines are absent, "'livl- The different formula, liy com- 

 parison, give us a fundamental formula from which they have been 

 derived by reduction. This was probably ^\l\- 



The molars undergo, according; to the food, the greatest modification of 

 form. As a startins,' point the huiioJonl tooth of omnivorous mammals may be 

 taken, which has the crown with several blunt projections or cones. With 

 animal food (figs. 6oi, 60S) the cones are sharper and cutting (sccoJiml dentition 

 of carnivores and insectivorcs), and when the cutting angle becomes very sharp, 

 with a special prominence on the inner side, it is spoken of as a flesh or car- 

 nasial tooth. In vegetafiic feeders the cones become connected by crests (lopJis) or 

 arehalf-moon-shapcd (loplioJoiiloTSclciioJoii/). Since the cones and lophs become 

 in part worn awav and the grooves between them are filled with cement, there 

 arise broad grinding surfaces strengthened by the harder and more resistant 

 enamel of the cones and lophs; this extends inwards as folds from the outer 

 enamel wall of the tooth; the folds may become cut off and form islands of enamel 

 on the grinding surface {denies com] icali of ungulates). AVhen the folds extend 

 in regular order from the outside and inside and meet in the middle they form 

 numerous successive plates bound together by cement (compound teeth of 

 elephants, fig. 61S, and many rodents). 



I'aleontological investigation, with which the more recent emliryological 

 results are in accord, has shown that a great regularity prevails in the formation 

 of the cones of the molars. Triconodonl and tritubcrciilar teeth are recognized, in 

 which the three cones are either arranged in a line or in a triangle, as well as 

 multiliihcrcular teeth with more numerous cones irregularly arranged. The 

 triconodont type develops farther by the formation of secondarv cones. The 

 development of these occurs in different ways in molars and premolars. Since 

 the latter are the more simple, their distinction from the molars does not rest 

 alone upon the existence of a milk dentition, but upon structure as well. This 

 is important, because it happens that there are premolars which are not replaced 

 (marsupials, many insectivorcs and rodents) and, on the other hand, beneath 

 the molars the anlagcn of replacing teeth may be found. The latter fact shows 

 that the molars, strictly speaking, belong not to the permanent but to the milk 

 dentition. They are late in formation and are therefore jiarts of the first 

 dentition carried over into the second. 



The lips, bounding the opening of the mouth, differ from those of all other 

 vertebrates by their soft character and the mobility caused by special muscles. 

 The mouth cavity, which contains tongue and teeth, is sc]iarated from the next 

 division of the alimentary tract, the pharynx, by the Kvida. The pharvn.x 

 narrows behind into the oesojihagus, the entrance of which into the stomach is 

 marked by a constricting cardia. At its other end the stomach has a similar 

 constrictor, the pylorus, sei)arating it from the intestine. In the latter small 

 and large intestines (the latter consisting of colon and rectum) are differentiated 

 by the diameter of the lumen. The small intestine opens laterally into the 

 colon and at the junction arises a blind diverticulum, the arciim, which is small 

 in mammals with animal food, but in herbivores (especially rodents) is always 

 large and forms a conspicuous part of the alimentary tract. The vermiform 

 appendix (primates, rodents) is a narrower part of the ca-cum with numerous 

 lymph follicles. Three pairs of salivary glands emptv into the mouth, the liver 

 and pancreas into the small intestine (duodenum). 



