The Integumentary System 87 



The vertebrate tooth can be seen to be structurally allied to the 

 placoid scales of the sharks. The projecting portion of dentine and 

 the internal pulp cavity are similar to those of a typical tooth. The 

 actual transition from placoid scales to fairly simple teeth may be 

 seen in the shark's jaw. 



Most vertebrates have simple conical teeth which in many are con- 

 stantly being replaced ; thus lower vertebrates have an indefinite num- 

 ber of teeth. Mammals, however, usually have a definite, limited num- 

 ber. The teeth in this latter group are primarily associated with the 

 grasping, grinding, and cutting of food. Often they are used as de- 

 fensive weapons. Birds which have eliminated all weighty materials 

 have no teeth. 



Many of the lower vertebrates have all their teeth exactly alike 

 in shape and structure. Such a condition is termed homodont ; this con- 

 trasts with the condition found in most mammals where the teeth are 

 unlike. This latter condition is referred to as heterodont. 



Typically a mammal has four types of teeth : the anteriormost ones 

 are the chisel-shaped incisors; the single canine is next in line and is 

 a long stout tooth with a sharp point ; following this are the premolars 

 with their large grinding surfaces; and finally the molars which have 

 even more complex crown surfaces. The development or regression of 

 the various teeth in different groups is closely related to the food habits 

 of the particular animal. The carnivores, for example, have well- 

 developed canine teeth for killing prey and tearing meat; rodents have 

 well-developed incisors useful for gnawing; cattle have huge molars 

 for chewing plant materials; and human beings have all types equally 

 well-developed, reflecting the fact that they eat everything. Much 

 can be learned of the food habits of a mammal simply by studying its 

 teeth. 



Since the number and types of teeth vary in different mammals, 

 a shorthand method has been developed for expressing it. This is 

 known as the tooth formula. The number of teeth of each type on each 



2-1-2-3 

 half of the jaw is indicated in the following manner: • This 



represents the tooth formula of an adult man and shows that there are 

 two incisors, one canine, two premolars, and three molars on each side 

 of both the upper and lower jaws, making a total of 32 teeth. The 



