CHAPTER VI 

 ALIMENTARY TRACT AND ADJACENT GLANDS 



Mouth. Just caudad of the incisors there is upon either 

 side a cartilaginous thickening of the cheek, constituting 

 the oral sphincter pad. These are in no degree muscular, 

 but they project from the ental surface of the cheek suffi- 

 ciently and are so situated that the two effect complete 

 closure of the mouth when the buccinator muscles are in 

 normal tone, thus excluding undesirable particles of matter 

 from the mouth. The more ventral portion of these pads 

 is sparsely covered with short hairs, corresponding to the 

 covering upon the integument of the lips, while the dorsal 

 portion is invested with mucous membrane. 



Beginning at the middle of the maxillary diastema and 

 extending upon the inner cheek for about three millimeters 

 caudad is a roughly circular, oral, bristle area upon either 

 side. This, in Homodontomys, is not raised into an emi- 

 nence and it is covered with a large number of fine bristles 

 two or three millimeters long. These incline towards the 

 gullet. A narrow band covered with fine hairs also extends 

 from this along a fold in the cheek to the corner of the lips. 

 In Neotoma this bristle area is shghtly raised, and in Teo- 

 noma, still more so, constituting a marked, hairy pad. 



The significance of this bristle area is obscure. It may 

 be a remnant of former and greater speciaKzation of the part, 

 or may constitute the earlier stage of some adaptational 

 organ. It is of some use, however, in that the backward- 

 pointing bristles upon either side form a hairy barrier com-, 

 pletely across the oral cavity at this point, allowing the 

 free entrance of particles of food but hindering their egress. 



The ridging of the mucous membrane upon the roof of 



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