^^ PHYSIOLOGY. 



leeder requiring a more complicated dental apparatus ; the carni- 

 vora being provided with teeth of a simpler construction, but more 

 fitted for seizing and tearing their prey. In the omnivorous animal 

 we hnd both the cutting and grinding teeth. In birds there are no 

 teeth ; and mastication, properly so called, is effected in the stomach 

 a portion of which (the gizzard) acquires a great increase of mus- 

 cular power, and is lined by a dense cuticle, and thus becomes a 

 powerful organ for triturating the food, the bird swallowing pieces 

 of flint or other hard substances to aid in the mechanical redaction 



Hu7iger and thirst. The want of soHd aliment is indicated by 

 the sensation of hunger,- and that of liquid by thirst. The former 

 of these sensations is referred to the stomach, and the latter to the 

 fauces • but, although these sensations may be caused by the condi- 

 tion of the parts mentioned, they are really indicative of the wants 

 of the system at large. 



The nerve which is instrumental in the sensation is probably the 

 vagus by Its gastric branches, but there is no reason for denyina to 

 the sympathetic nerve distributed to the stomach, some share in This 

 phenomenon; the latter probably conveys the wants of the system 

 to the stomach, while the former is the medium by which those 

 wants are expressed. Section of the vagus abates, if it does not 

 entirely arrest the sensation of hunger. The same is true of the 

 introduction of matters not alimentary into the cavity of the organ 



Thirst resuM^ from a peculiar state of the mucous membrane of 

 the digestive tube, but more especially of the mucous membrane of 

 the mouth and fauces, caused by the imperfect supply of liquid 

 1 hirst IS perhaps more immediately connected with the wants of the 

 general system than hunger, since the relief that is afforded by the 

 introduction of liquid into the stomach is immediate, and may be 

 fully accounted for by the instantaneous absorption of the fluid into 

 the veins. Any excess in the amount of fluid excretions will in- 

 crease this sensation, as is also the case when stimulating or irri- 

 tating articles of food have been used ; the purpose of this increase is 

 obviously to cause ingestion of fluid, by which they may be diluted 

 1 he process of digestion may be divided into the following staoes • 

 1st. Prehension of food; 2^. Mastication and insalivation ^ U 

 Deglutition ;^ 4th. Chymification ; 5th. Action of small i72testine': 

 olh. Defecation. 



Prehejision, or the taking of food into the mouth, is performed 

 mamly by the hand, assisted by the lips and cheeks, as well as the 

 anterior teeth and the tongue. 



^ Mastication has for its object the comminution of the food so that 

 It can be readily acted upon by the solvent juices of the stomach. 

 1 he contact of the solid food with the interior of the mouth excites 

 the act of mastication, performed by alternating contractions of the 

 muscles which pull the lower jaw upward, downward, backward, 



