

FUNCTIONS OF VEGETATIVE LIFE. 77 



of the fabric that are thus lost. The effects of the process of decay, when 

 uncompensated by that of renovation, are remarkably seen in cases of starva- 

 tion ; for it is a very constant indication of this condition, that the body exhales 

 a putrescent odour, even before death, and that it subsequently passes very 

 rapidly into decomposition. This, it may be considered, is the reason why a 

 constant supply of aliment is still required for the maintenance of every organic 

 structure, though it may have arrived at its full growth ; and it also affords 

 one source of explanation of the fact, that old people require less food than 

 adults, since their tissues are more consolidated, and thus become at the same 

 time unable to perform their usual actions with their pristine energy, whilst 

 their tendency to decomposition is less. In the growing state, however, an 

 additional important source of demand for food obviously exists, in the extension 

 which the tissues themselves are constantly receiving ; yet this, perhaps, does 

 not make so great a difference as it appears to do in the supply which is 

 requisite. For if the addition which is made by growth to the body in any 

 given time, be compared with the amount of exchange which has taken place 

 in the same time, the latter being judged of by the quantity of matter excreted 

 from the lungs, liver, kidneys, skin, &c., it will be found to bear but a very 

 small proportion to it, except during fetal life, when the growth is very rapid, 

 and a large proportion of the effete particles are brought to the maternal blood, 

 to be excreted from it. The real cause of the increased demand for nutriment 

 during the early part of life is rather this, that the tissues are far from having 

 acquired that firmness and consolidation 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 

 ( 430, 437), 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 ali- 

 ment, 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 qf 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.* 



85. 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 instinctive, resulting 

 merely from the impression made upon the fauces by the contact of the sub- 

 stance swallowed, which impression is conveyed to the medulla oblongata and 

 reflected back to the muscle ( 191). By these it is propelled down the oeso- 

 phagus ; 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 oesophagus 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 ( 194). In the stomach, the food is subjected to the gastric 

 secretion ; the chemical action of which, aided by the constantly elevated tem- 

 perature 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. Reason will hereafter appear for the belief that, up to this point, 

 no action peculiarly vital is immediately concerned in the reduction of the 

 food ; and that, if the physical conditions of the process could be exactly imi- 

 tated out of the body, the result would be precisely the same. The mixture 

 of the biliary and pancreatic secretions with the chyme thus produced, occa- 



7* 



