426 NUTRITION ANIMAL HEAT AND FORCE. 



is very like that of the medullary portion of the suprarenal capsules 

 (Grandry). 



It is difficult to classify organs, of the uses of which physiologists are en- 

 tirely ignorant; but in structure, the little bodies just described certainly 

 resemble the ductless glands. 



CHAPTER XIV. 



NUTRITION ANIMAL HEAT AND FORCE. 



Nature of the forces involved in nutrition Life, as represented in development and nutrition Substances 

 which pass through the organism Metabolism Substances consumed in the organism Conditions 

 which influence nutrition Animal heat and force Estimated quantity of heat produced by the body 

 Limits of variation in the normal temperature in man Variations with external temperature Varia- 

 tions in different parts of the body Variations at different periods of life etc. Influence of exercise 

 etc., upon the heat of the body Influence of the nervous system upon the production of animal heat 

 (heat-centres) Mechanism of the production of animal heat Equalization of the animal temperature 

 Relations of heat to force. 



NUTRITION proper, in the light in which it is proposed to consider it in 

 this chapter, is the process by which the physiological wear of the tissues and 

 fluids of the body is compensated by the appropriation of new matter. All 

 of the physiological operations that have thus far been described, including 

 the circulation of the blood, respiration, alimentation, digestion, absorption 

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

 great end, to which all of the functions enumerated 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 gen- 

 eral process, however, have been carefully studied, and certain important 

 positive results have been attained ; but there is really no more satisfactory 

 explanation of the nature of the causative force of nutrition to be found in 

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



The blood contains all the matters that enter into the composition of the 

 tissues and secretions, either identical with them in form and composition, 

 as is the case in most of the inorganic matters, or in a condition .which 

 admits of their transformation into the characteristic constituents of the tis- 

 sues, as in the organic substances proper. These matters are supplied to the 

 tissues, in the required quantity, through the circulatory apparatus ; and oxy- 

 gen, which is immediately indispensable to all the operations of life, is intro- 

 duced by respiration. The great nutritive fluid,, being constantly drawn upon 

 by the tissues for materials for their regeneration, is kept at the proper stand- 

 ard by the introduction of new matter into the system in alimentation, its 

 elaborate preparation by digestion, and its appropriation by the fluids by 

 absorption. Many of these processes require the action of certain secretions 



