386 



ANATOMY OF VERTEBRATES. 



299 



In Rodentia the sealpriform incisors are commonly more or 

 less exposed in front of the mouth ; and, as their office is to re- 

 duce the food to small bits, the mouth is small. A groove running 

 thereto from the nostrils divides the upper lip, conspicuously so 

 in the species which has suggested for this modification in other 

 animals the name ( hare-lip.' But in a few Rodents, e.g. Mole- 

 rats ( Orycteropus, Spalax), the undivided upper lip surrounds the 

 bases of the huge pair of incisors by a kind of hairy sheath, and the 

 lower lip is similarly modified in relation to the prominent lower 

 incisors. The hairy integument is continued or reflected within the 

 mouth in some degree in most Rodents. In the Paca ( Cozlogenys) 

 it is continued along the inside of the cheeks, with an accession of 

 glandular follicles ; then, losing the hair, it lines a large cavity 

 formed by the singular expansion of the zygomatic process of the 



maxillary and by the malar 

 bone, vol. ii. fig. 237, 21, 26. 

 Some Rodents have dupli- 

 catures of the buccal mem- 

 brane, outside the zygomata, 

 and capable of expansion, 

 for storage and conveyance 

 of alimentary substances. 

 Fig. 299 shows these 

 ' cheek-pouches ' in Geomys 

 **» our sarins, everted and m- 



Cheek-pouches of the Canada Rat {Geomys bursariws). n . -\ 1 



flated : a more natural view 

 of this buccal appendage is given in the dissection, fig. 300, of 

 the head of an African pouched rat. In this species (Sacco- 

 stomus lapidarius, Peters) an orifice at the angle of the mouth 



leads to the pouch, widening from 

 the orbit to the lower border of the 

 mandible, and reaching back as far 

 as beneath the ear. In the Ham- 

 ster ( Cricetus) the wide orifice of 

 the pouch is just within the com- 

 missure of the short lips : the bag 

 itself extends along the side of the 

 head to the neck. The orifice has a 

 sphincter, and from this there diverge longitudinal fasciculi back- 

 ward over the wall of the cavity, which is also provided with 

 fibres from the dorso-lateral panniculus carnosus : these tending to 

 retract the pouch-walls, while the others draw them forward, and 

 both combining to empty the pouch. Saccomys and Spermophilus 



300 



Cheek -pouch (Saccostomns). lxxxiv 



