PREHENSION OP SOLID FOOD. 39 



is said to be folded into a tubular shape for that purpose. 

 But suction among the mammalia is almost always performed 

 by the muscles of the lips and cheeks, aided by the move- 

 ments of the tongue, which, when withdrawn to the back of 

 the cavity, acts like the piston of a pump. In the lamprey 

 this hydraulic action of the tongue is particularly remarka- 

 ble. Many quadrupeds, however, drink by repeatedly dip- 

 ping their tongue into the fluid, and quickly drawing it into 

 the mouth. 



2. Prehension of Solid Food. 



WHEN the food consists of solid substances, organs must 

 be provided; first, for their prehension and introduction into 

 the mouth; secondly, for their detention when so introduced; 

 and thirdly, for their mechanical division into smaller frag- 

 ments. 



Of those instruments of prehension which are not portions 

 of the mouth itself, and which form a series of variously 

 constructed organs extending from the tentacula of the po- 

 lypus to the proboscis of the elephant, and to the human arm 

 and hand, some account has already been given in the his- 

 tory of the mechanical functions: but, in a great number of 

 instances, prehension is performed by the mouth, or the 

 parts which are extended from it, and may be considered as 

 its appendices. The prehensile power of the mouth is de- 

 rived principally from the mechanical form and action of 

 the jaws, which open to receive, and close to detain the 

 bodies intended as food; and to this latter purpose, the teeth, 

 when the mouth is furnished with them, likewise materially 

 contribute, although their primary and more usual office is 

 the mechanical division of the food, by means of mastication, 

 an action in which the jaws, in their turn, co-operate. Ano- 

 ther principal purpose effected by the ja\vs is that of giving 

 mechanical power to the muscles, which, by acting upon 

 the sides of the cavity of the rnouth, tend to compress and 



VOL. II. 12 



