768 SPECIAL PHYSIOLOGY. 



EXCRETION. 



The characters which distinguish excretion from secretion proper, 

 have already been detailed (pp. 751-3). Its products, like those of 

 secretion, are fluid or gaseous, at least in the human body, although 

 semisolid or solid urine occurs in Birds and Reptiles. The term ex* 

 creta, is also commonly applied to the solid materials ejected from the 

 intestinal canal, as the residuum of the digestive process. These, 

 however, are only partly excreted substances, such as unabsorbed bili- 

 ary and other products more or less changed, substances thrown out 

 by the intestinal glands, and undigested mucus and epithelium. The 

 greater portion of the mass, however, consists of undigested food, such 

 as elastic tissue, sarcolemma, the walls of vegetable cells, spiral ducts, 

 and woody fibre. 



The fluid excretions are those eliminated by the kidneys and the 

 skin, the former excretion being the more complex. The exhalation 

 of carbonic acid from the lungs, is an excretory process more imme- 

 diately necessary to life than any other. The lungs may be regarded 

 as excretory glands, and the carbonic acid expelled from them as a 

 gaseous excretion ; but the specialty of this process, its association 

 with the absorption of oxygen, and its peculiar mechanism, render it 

 necessary to consider the entire function separately, under the head 

 of Respiration. 



Renal Excretion. 



The urine, excreted by the kidneys, is the most perfect example of 

 a fluid excretion given off by the animal economy ; its various con- 

 stituents exist preformed in the blood ; they are, moreover, highly 

 oxidized nitrogenous products of the decomposition of the albuminoid 

 tissues, and of the albuminoid constituents of the blood. They are 

 destitute of organization, and incapable of it ; neither can they be 

 made, in animals, to undergo an ascensive chemical metamorphosis 

 fitting them for nutrient purposes ; they are of no further use in the 

 organism, and, indeed, if retained, are highly injurious to it. Hence 

 they are destined to be, once for all, separated, or excreted from it, 

 and to be, as soon as possible, entirely discharged from the body. It 

 facilitates this end that they are chiefly crystalloid bodies, and there- 

 fore easily dialyzable. It is provided, moreover, that a very large 

 proportion of the blood should pass through the organs by which these 

 substances are eliminated, for the quantity sent through them in twenty- 

 four hours amounts to nearly 2000 Ibs. (Brown-Se'quard) ; so that all 

 the blood in the body may pass through them 150 times in that period. 

 Lastly, the excretion of urine is continuous or incessant. 



The Kidneys. 



The kidneys are two dense, firm, dark-red, solid, but fragile gland- 

 ular organs, situated at the back part of the abdominal cavity, in the 

 lumbar region, one at each side of the vertebral column, on a level 

 with the last dorsal and the two or three upper lumbar vertebrae, and 

 reaching from the eleventh rib to near the crest of the hip bone. The 

 right kidney, owing to the proximity of the liver on that side, is about 



