274 OF FOOD AND THE DIGESTIVE PROCESS. 



ducts of the secretion of the Peyerian and other glandulse, with which 

 the mucous lining of the lower intestines is studded. Many of the lower 

 animals are furnished, at the part where the small intestine enters the 

 large, with a caecum, resembling that which in Man is termed the vermi- 

 form appendage of the caecum, but greatly exceeding it in size. Some- 

 times we find two caeca instead of one ; and these are much prolonged, 

 so as to* form tubes of considerable length. It has been ascertained 

 that, in herbivorous animals, a distinctly acid secretion is formed by the 

 caecum, during the digestive process ; and there is reason to believe, 

 that the food there undergoes a second process, analogous to that to which 

 it has been submitted in the stomach, and fitted to extract from it any 

 undissolved alimentary matter which it may still contain. There is no 

 reason to believe, however, that any such process takes pkce in Man, 

 whose real caecum is rudimentary, the part of the intestine which has 

 received the name, being merely the dilated commencement of the colon. 

 The act of Defecation, by which the excrementitious matter is discharged, 

 has been already noticed ( 462) ; the Absorption of nutritive matter 

 will be treated of in the succeeding Chapter. 



5. Of Hunger, Satiety, and Thirst. 



482. The want of solid aliment is indicated by the sensation of 

 Hunger ; and the deficiency of fluid by that of Thirst. On the other 

 hand, the presence of a sufficiency of food or liquid in the stomach is 

 indicated by the sense of Satiety. These sensations are intended as 

 our guides, in regard to the amount of aliment we take in. What is 

 the real seat of these sensations, and on what conditions do they de- 

 pend ? 



483. The sense of Hunger is referred to the stomach, and seems im- 

 mediately to depend upon a certain condition of that organ ; but what 

 that condition is, has not yet been precisely ascertained. It is not pro- 

 duced by mere emptiness of the stomach, as some have supposed ; for, 

 if the previous meal have been sufficient, the food passes entirely from 

 the cavity of the stomach, before a renewal of the sensation is felt. It 

 cannot be due to the action of the gastric fluid upon the coats of the 

 stomach themselves ; because this fluid is not poured into the stomach, 

 except when the production of it is stimulated by the irritation of the 

 secreting follicles. It has been attributed to distention of the gastric 

 follicles by the secreted fluid; but there is no evidence that the fluid is 

 secreted before it is wanted ; and, moreover, it is well known that mental 

 emotion can dissipate in a moment the keenest appetite, and it is diffi- 

 cult to imagine how this can occasion the emptying of the follicles. 

 Perhaps the most satisfactory view is that, which attributes the sense of 

 hunger to a determination of blood to the stomach, preparing it for the 

 secretion of gastric fluid ; since this is quite adequate to account for the 

 impression made upon the nerves ; and it accords with what has just 

 been stated of the influence of mental emotions, since we know that 

 these have a powerful effect upon the circulation of blood in the minute 

 vessels ( 603). 



484. Although the sense of Hunger is immediately dependent, in 



