486 NUTRITION". 



It is difficult to classify organs, of the function of which we are entirely ignorant; but 

 the structure of the little bodies just described certainly resembles that of the ductless 

 glands. "We have indicated their anatomy merely to show that their function is probably 

 analogous to that of other organs of the same class. 



CHAPTER XV. 



NUTRITION ANIMAL HEAT. 



Nature of the forces involved in nutrition Definition of vital properties Life, as represented in development and 

 nutrition Principles which pass through the organism Principles consumed in the organism Development of 

 power and endurance by exercise (training) Formation and deposition of fat Conditions under which fat exists 

 in the organism^Physiological anatomy of adipose tissue Conditions which influence nutrition Products of 

 disassimilation Animal heat Limits of variation in the normal temperature in man Variations with external 

 temperature Variations in different parts of the body Variations at different periods of life Diurnal variations 

 Eelations of animal heat to digestion Influence of defective nutrition and inanition Influence of exercise, 

 mental exertion, and the nervous system, upon the heat of the body Sources of animal heat Connection of the 

 production of heat with nutrition Seat of the production of animal heat Eelatiocs of animal heat to the different 

 processes of nutrition Relations of animal heat to respiration Exaggeration of the animal temperature in par- 

 ticular parts after division of the sympathetic nerve and in inflammation Intimate nature of the calorific pro- 

 cesses Equalization of the animal temperature. 



NUTRITION proper, in the light in which we propose to consider it in this chapter, 

 is the process by which the physiological decay of the tissues and fluids of the body is 

 compensated by the appropriation of new matter. All of the physiological processes 

 that we have thus far studied, including circulation, respiration, alimentation, digestion, 

 absorption, and secretion, are to be regarded as means directed to a single end ; and 

 the great function, to which all the others are subservient, is the general process of 

 nutrition. 



The nature of the main forces involved in nutrition, be it in a highly-organized part, 

 like the brain or muscles, or in a tissue called extra- vascular, like the cartilages or nails, is 

 unknown. The phenomena attending the general process, however, have been studied 

 most carefully, and certain important positive results have been attained ; but we really 

 find no more satisfactory explanation of the nature of the causative force of nutrition in 

 the doctrines of to-day than in the speculative theories of the ancients. 



We can hardly realize the vast extent of the problem of nutrition from a review of the 

 functions which we have already considered. We have seen that the blood contains all 

 the elements that enter into the composition of the tissues and secretions, either identical 

 with them in form and composition, as is the case with the inorganic principles, or in a 

 condition which allows of their transformation into the characteristic principles of the 

 tissues, as we see in the organic substances proper. These materials are supplied to 

 the tissues, in the required quantity, through the' circulatory apparatus; and oxygen, 

 which is immediately indispensable to all the operations of life, is introduced by respira- 

 tion. The great nutritive fluid, being constantly drawn upon by the tissues for materials 

 for their regeneration, is kept at the proper standard by the introduction of new matter 

 into the system in alimentation, its elaborate preparation by digestion, and its appropria- 

 tion by the fluids by absorption. Many of these processes require the action of cer- 

 tain secretions. 'The introduction of new matter, so essential to the continuance of the 

 phenomena of life, is demanded, on account of the change of the substance of the tissues 

 into what we call effete matter ; and this is discharged from the animal organism, to be 

 appropriated by vegetables, and thus maintain the equilibrium between these two great 

 kingdoms in Nature. 



What is it that causes the parts of a living animal organism to undergo change into 



