r :HE DIGESTIVE AND RESPIRATORY SYSTEM 275 



<:ome under the immediate control of the masticatory muscles. 

 In the Cetacea alone the teeth are all alike (homodont) and of 

 the simple conical type, and here it may be considered certain 

 that this condition is a secondary one and that the teeth have 

 lost the usual complexity through disuse; since an aquatic 

 animal cannot chew and employs its teeth merely for the pur- 

 pose of holding and retaining the food. In all other mammals 

 the teeth are heterodont, and possess several distinct forms, 

 which have been conveniently divided into three types, incisors, 

 canines, and molars, with a possible subdivision of the last 

 into premolars and molars proper. 



Of these types the incisors are the most anterior and con- 

 sist in the upper jaw of those borne by the premaxillary bones 

 and in the lower jaw of those opposite the former. There may 

 be five of these in each upper half jaw, and four in each lower 

 half, but the usual number is three or two. Corresponding 

 to the most frequent function of teeth thus situated their 

 typical shape is that of chisels for cutting or biting, 

 a form easily derived from the primitive conical forms by 

 a flattening in a labio-lingual direction. Beyond these 

 there is in each half jaw a single canine, typically in the 

 form of a pointed cusp elongated beyond the level of the 

 other teeth and best preserving the primitive shape. The can- 

 ines of the lower jaw lie anterior to those of the upper jaw and 

 in the case of elongated canines slip past one another. As 

 their chief policy is that of piercing and tearing, they become 

 reduced or are entirely wanting in animals in which this func- 

 tion is superfluous. The remaining teeth may be classed to- 

 gether as molars, or cheek-teeth, or a distinction may be 

 made in most cases between an anterior group of premolars, 

 in which the first teeth are usually replaced by a second set, 

 and a posterior group of true molars, which develop later than 

 the rest and are not replaced. That this distinction is in part 

 an artificial one and often difficult or impossible of application 

 is shown by the study of the subject of replacement, consid- 

 ered below. Of premolars the greatest number that occurs 

 in mammals is four in each half-jaw, of definite molars, five. 



