364 GENERAL VIEW OF THE HUMAN FUNCTIONS. 



that its renovation may be effected on a somewhat different scale ( 130, 131). 

 And thus it happens that children require a much larger amount of food in pro- 

 portion to their bulk, than that which suffices for adults. On the other hand, 

 in old persons, the life of each part is comparatively slow; its vital operations 

 are deficient in activity; and the processes of waste and the demand for food are 

 proportionally retarded ( 133). 



375. But another and most important source of demand for food, in Man, 

 and warm-blooded animals generally, arises out of the requirements of the com- 

 bustive process, whereby the Heat of the body is maintained. This demand 

 will vary, cseteris paribus, with the amount of heat to be generated, which bears 

 a direct proportion to the depression of the external temperature, the standard 

 of the body itself being fixed. Hence external Cold comes to be a source of 

 demand for food; whilst artificial warmth may be made to take the place of the 

 nourishment otherwise required for this purpose; as was shown by the remark- 

 able experiments of Chossat, hereafter to be referred to (CHAPS, vn. and xin.). 

 But if the amount of exercise taken be very considerable, especially in warm 

 climates, where the demand for the production of Heat is reduced to its minimum, 

 a sufficient amount of pabulum for the respiratory process may be provided by 

 the disintegration of the nervo-muscular apparatus, without any special supply 

 being required. 



376. The demand for food is increased by any cause which creates an unusual 

 drain or waste in the system. Thus an extensive suppurating action can be 

 sustained only by a large supply of highly nutritious food. The mother, who 

 has to furnish the daily supply of milk, which constitutes the sole support of 

 her offspring, needs an unusual sustenance for this purpose. And there are 

 states of the s^tem, in which the solid tissues seem to possess an abnormal 

 tendency to decomposition, and in which an increased supply of aliment is there- 

 fore required. This is the case, for example, in Diabetes; one of the first symp- 

 toms of which disease is the craving appetite, that seems as if it would be never 

 satisfied. And there can be no doubt that, putting aside all the other circum- 

 stances which have been alluded to, there is much difference amongst individuals, 

 in regard to the rapidity of the changes which their organism undergoes, and 

 the amount of food consequently required for its maintenance. 



377. The want of solid aliment is made known by the feeling of Hunger; 

 and that of liquids, by the feeling of Thirst. These feelings, as will be shown 

 hereafter (CHAP. vn. SECT. 2), are but secondarily dependent upon the state 

 of the stomach; and may be considered, in the state of health, as tolerably 

 faithful indications of the wants of the body at large. They become the stim- 

 ulants to mental operations, having for their object the gratification of the de- 

 sire by the acquisition of food. In the state of Infancy, the actions which they 

 prompt are obviously automatic; that is, they are performed in direct respond- 

 ence to the appropriate stimulus, and do not involve any idea of purpose or 

 object on the part of the being which executes them. But, in all succeeding 

 periods of life, they are purely voluntary ; being performed with a designed or 

 purposive adaptation of means, to ends which are clearly before the conscious- 

 ness. The reception of food into the mouth, and its preparation by the acts 

 of mastication and insalivation, would seem rather to belong to the consensual 

 or " sensori-motor" class of movements; being performed quite independently 

 of the will, whenever that power is in abeyance, or is differently directed. By 

 these movements, the aliment is brought within reach of the pharyngeal mus- 

 cles, whose contraction cannot be effected by the will, but is purely reflex, or 

 " excito-motor," resulting merely from the conveyance to the Medulla Oblongata 

 of the impression made upon the fauces by the contact of the substance swal- 

 lowed, and from the reflexion of an influence excited by that impression, back 

 to the muscles. By these it is propelled down the oesophagus; and, after their 



