THE STRUCTURE AND ACTIVITIES OF MAMMALS 405 



ure 275. The enamel (in black) is the outer hard substance; 

 the dentine (horizontal lines) constitutes the largest portion of 

 the tooth; and the cement (dotted) usually covers the part of the 

 tooth embedded in the tissues of the jaw. The central pulp cav- 

 ity of the tooth contains nerves, blood vessels, and connective 

 tissue. Teeth have an open pulp cavity during growth (Fig. 

 275, II), which in some cases continues throughout life (Fig. 275, 1). 



The teeth of fishes, reptiles, and amphibians are usually all 

 similar, but in mammals there are commonly four kinds in each 

 jaw: (i) the chisel-shaped 

 incisors in front (Fig. 276, 

 i 2), (2) the conical canines 

 W> (3) the anterior grind- 

 ing teeth or premolars (pm i 

 -pm 4), and (4) the pos- 

 terior grinding teeth or mo- 

 lars (m i}. 



In most mammals the first 

 set of teeth, known as the 

 milk dentition, is pushed 

 out by the permanent teeth, 

 which last throughout the 

 life of the animals. The 

 milk molars are followed by the premolars, but the permanent 

 molars have no predecessors. 



The relation of the form of the teeth to the food habits of the 

 animal may be shown by the following examples. The dol- 

 phins have a large number of sharp, conical teeth adapted for 

 capturing fish; the carnivorous animals, like the dog (Fig. 276), 

 are provided with large canine teeth for capturing and killing 

 their prey, small and almost useless incisors, and molars with 

 sharp edges for cutting or crushing; herbivorous mammals, 

 like the ox, possess broad incisors for biting off plants, no canines, 

 and large grinding molars; gnawing mammals, like the rabbit, 

 have incisors that grow throughout life, but are worn down by 



FIG. 276. Teeth of dog. 



i 2, second incisor ; c, canine ; pm 1, 

 pm 4, first and fourth premolars ; m 1, first 

 molar. (From Shipley and MacBride.) 



