1128 



DIGESTIVE SYSTEM 



wide variation among the various forms. In most mammals, as in man, the number of sets of 

 teeth has been reduced to two, or diphyodont dentition, with only traces of an earlier (pre- 

 lacteal) and also a later (post-permanent) set. In some mammals (monotremes, cetacea) 

 the dentition has been reduced to a single set, mono-phyodont, while in birds all except rudi- 

 mentary traces of dentition have been lost. 



*As may be further observed in fig. 887, the primitive teeth are of a recurved conical form, 

 and serve primarily for grasping and holding the food. The specialization of the teeth for 

 purposes of mastication is in general a secondary acquisition amongst higher vertebrates. 



It is also noteworthy that the primitive teeth, as found among nearly all forms below the 

 mammals, are practically alike in form, i. e., homodont. Among mammals, however, there is 

 a marked specialization of the teeth, or heterodont dentition. The mammalian teeth are usually 

 differentiated into four distinct classes, incisors, canines, premolars and molars, similar to those 

 found in man. 



The typical or complete mammalian dentition, however, contains a larger number of teeth 

 than found in man, and is represented by the formula 



• 31 43 



^3' ^r P"^ 4' "^3=44. 



Thus it is evident that there has been a reduction in the incisors and premolars in the human 

 species, and there has been considerable discussion of the question as to which teeth of the 



Fig. 887. — Section through Lower Jaw of Dog-fish, Showing the Development of the 

 Oral Teeth, and the Transition to Dermal Teeth. M, mandible. (After Gegenbaur.) 



Intermediary forms — 



Oral tooth 



Skin of lower jaw 



Dermal tooth 



, -- Developing tooth 



-- Dental epithelial ridge 



. Epithelium of oral mucosa 



lij^.'i .Anlage of tooth 



primitive series have been lost. This reduction in the number of teeth is probably correlated 

 with the general reduction in the jaws, which are relatively much larger and stronger in the 

 savage races and lower animals. The third molar, or wisdom tooth, is probably now on the 

 road to extinction, due to a continuation of the same evolutionary process. 



Another interesting problem, concerning which there has been much speculation, is the 

 origin of the multicuspidate mammalian molar. It has clearly been derived from the primitive 

 conical type of the homodont dentition, but as to the method of evolution there is a difference 

 of opinion. According to one view (the 'concrescence' theory), the molar has been derived 

 by a process of fusion, each cusp representing a primitive conical tooth. Another view (the 

 'differentiation' theory) is that the molar represents a single primitive tooth, upon the crown 

 of which the various cusps have been differentiated. According to a third view, which is a 

 comj)roniise, the tritubercular (tricuspid) form of tooth, which is that found in the earliest 

 fossil inamiiials, was derived by a process of <u)iicrescence of three primitive teeth, while from 

 this tricuspid form the multicuspidate molar has been derived by a process of differentiation. 



THE PHARYNX 



Tlio pharynx is a vortical, tul)ular passafj^e, flattened antoro-posteriorly, and 

 extending from the base of the eraniiini above to tlie beginning of the oesophagus 

 below. Posteriorly, it is in contact with the bodies of the upper six cervical verte- 

 brse. Laterally, it is in relation with the internal and common carotid arteries, 



