98 GENERAL PHYSIOLOGY. 



that of nutrition ; in the former, the fluid material elaborated from 

 the blood, escapes on to the external or internal surfaces of the body ; 

 whilst in the latter it is retained within the body in the more solid 

 form of tissue. Besides those just mentioned as associated with the 

 alimentary canal, other glands, such as the lachrymal and mammary 

 glands, exist, the secretions of which fulfil special offices in the econ- 

 omy. In addition to the continued alteration of the blood produced 

 by its subservience to so great a variety of nutritive processes, by the 

 loss of stimulating material conveyed to the muscular and nervous 

 tissues, by the varied process of secretion, and by the operations con- 

 nected with nutritive absorption and sanguification, the blood, as we 

 have seen, is made the vehicle for the reception of the waste material 

 of the disintegrated tissues, which, dissolved in the residual plasma 

 exuded amongst their ultimate structural elements, is, at least in part, 

 reabsorbed into the circulating current. These effete matters, if per- 

 mitted to accumulate in the blood, poison it, and render it unfit for 

 the stimulation of the nervous and muscular tissues, for the proper 

 nutrition of the tissues generally, and for the purposes of healthy se- 

 cretion. Accordingly, another function is added to the nutritive vege- 

 tative functions of the animal economy, named excretion, by means of 

 which the blood is enabled to get rid of these effete materials through 

 the action of certain emunctory organs, named the excreting glands, 

 of which the chief are the kidneys, the cutaneous sweat glands, and 

 the lungs. The liver and the intestinal mucous membrane, moreover, 

 also assist in this excretory function. By means of the urinary, cuta- 

 neous, and pulmonary excretions, and of the solid excreta from the 

 alimentary canal, all the products of the decomposition of the tissues 

 are regularly removed ; and as these tissues are as constantly reno- 

 vated from the blood, and the blood itself from the food, there exists 

 a balance in the nutritive actions of the living economy, and a corre- 

 spondence between the daily quantity of food consumed, and the daily 

 amount of the vito-chemical nutritive changes occurring in the body. 



Of the various excretory processes, there is one, viz., the elimina- 

 tion of the carbonic acid from the lungs, which is distinguished from 

 the rest by its being associated with another process equally essential 

 to animal life, viz., the introduction of oxygen into the blood and tis- 

 sues of the living animal. This is accomplished in breathing, the 

 characteristic act of that most important function, respiration. After 

 the reception of food into the body, all the ensuing nutritive processes 

 which we have described above, are hidden or concealed from observa- 

 tion ; but the process of breathing is one which is externally mani- 

 fested. The animal under observation, indeed, is seen to breathe; the 

 sides of its thorax expand and contract, and it alternately draws in 

 and expels air from the interior of its frame. The air enters through 

 the nostrils, and also sometimes through the mouth, into the throat or 

 pharynx, and thence through the larynx, windpipe and its subdivisions 

 into the lungs, and then it is again expelled from those organs through 

 the same air-passages. The former act is called inspiration, the latter 

 expiration. The air which escapes from the lungs has not the same 

 chemical composition as when it entered them ; for, within those or- 



