226 GENERAL VIEW OF THE FUNCTIONS. 



this, that the tissues are far from having acquired that firmness and consoli- 

 dation which they gain at adult age; and that they are, therefore, more prone 

 to decomposition, at the same time that their vital activity is greater, as is well 

 known to be the case. The feeling of hunger or desire for food originates, 

 we shall hereafter find reason to believe (Chap. X., Sect. 1), not so much in 

 the stomach itself, as in the system at large; of whose condition, in regard to 

 the requirement of an increased supply of aliment, it may, during the state of 

 health, be considered as a pretty faithful index. The same may be said of 

 thirst. The feeling of hunger, then, is the stimulus to the mental operations, 

 which have for their object the acquisition of food; whether these be of a 

 voluntary or of a purely instinctive kind. In Man they are obviously the 

 former, during all but infant life. 



270. The food received into the mouth, and prepared there by the acts of 

 mastication and insalivation (the movements concerned in which are dependent 

 upon the brain, and can only be performed when it is in a condition of some 

 activity), is brought by them within reach of the pharyngeal muscles, whose 

 contraction cannot be effected by the will, but is purely excito-motor, result- 

 ing merely from the impression made upon the fauces by the contact of the 

 substance swallowed, which impression is conveyed to the medulla oblongata 

 and reflected back to the muscles ( 383). By these it is propelled down 

 the oesophagus ; and, after their action has ceased, it is taken up (as it were) 

 by the muscular coat of the oesophagus itself, and conveyed into the stomach. 

 How far the movements of the lower parts of the oasophagus and of the 

 stomach are in Man dependent upon reflex action, is uncertain ; the facts 

 which have been ascertained on this point, by experiment on animals, will be 

 detailed in their proper place ( 390). In the stomach the food is subjected 

 to the gastric secretion ; the chemical action of which, aided by the con- 

 stantly-elevated temperature of the interior of the body, and by the continual 

 agitation effected by the contractions of the parietes of the organ, effects a 

 more or less complete solution of it. The mixture of the biliary and pan- 

 creatic secretions with the chyme thus produced, occasions a separation of its 

 elements into those adapted for nutrition, and those of which the character is 

 excrementitious; and this separation can scarcely be regarded in any other 

 light than as a chemical precipitation. By the agency of the biliary secre- 

 tion, moreover, certain elements of the food that would otherwise be rejected, 

 are reduced to a form in which they can be absorbed. The nutritious por- 

 tion is taken up by the Blood-vessels and by the Absorbent vessels (or Lac- 

 teals), which are distributed on the walls of the alimentary canal; whilst the 

 remainder is propelled along the intestinal tube by the simple contractility of 

 its walls, undergoing at the same time some further change, by which the 

 nutritive materials are still more completely extracted from it. And at last, 

 the excrementitious matter, consisting not only of a portion of the food 

 taken into the stomach, but also of part of the secretion of the liver, and of 

 that of the mucous surface of the intestines and of their glandulrc, is avoided 

 from the opposite extremity of the canal, by a muscular exertion, which is 

 partly reflex, like that of deglutition, but is partly voluntary, especially (as it 

 would appear) in Man. 



271. There seems no doubt that fluid containing saline, albuminous, or 

 other soluble matters, may be absorbed by the Blood-vessels, with which the 

 mucous membrane of the alimentary canal is so copiously supplied ; and this 

 simple process of Imbibition probably takes place according to the physical 

 laws of Endosmose. But the Selection and Absorption of certain nutritious 

 elements appear to be performed, not by vessels, but by the growth and de- 

 velopment of cells ( 181); which, by their subsequent disintegration, give it 

 up to the Lacteals. The absorbed fluid, which now receives the name of 



