448 SECRETION. [CHAP. xxxn. 



which escape in the form of faeces; and, allied to these, is the 

 carbonic acid, which is eliminated, without the intervention of a 

 true secreting tissue, directly from the capillaries of the lungs. If 

 these matters were retained, they would accumulate in the blood 

 and tissues, and prove, sooner or later, incompatible with life; and 

 even a simple delay or retardation in the process of their removal, 

 to whatever cause it may be attributable, must be attended with 

 grave consequences to health. It is, therefore, impossible to over- 

 estimate the importance of distinguishing these products from 

 other secretions, and of acquiring the habit of considering them 

 under one common head. They are known as the excrementitious 

 secretions , or, simply as excretions. 



The kidneys are organs through which a large portion of the 

 blood is constantly flowing, and in which the general mass of that 

 fluid becomes so far purified as to be freed from certain waste ma- 

 terials which have just been thrown into it by the muscular and 

 other tissues. The blood which leaves the kidneys, though ren- 

 dered venous by the nutrition of the glandular tissue itself, is more 

 free than before to receive the refuse of the other parts. Hence 

 the kidneys must be regarded as a depurating organ, subservient to 

 the functions of other parts of the frame. To prove this, it is only 

 necessary to extirpate the kidneys from an animal, or to see their 

 function arrested by disease : in either case, the blood and tissues 

 become more and more loaded by the urinary matters which continue 

 to be formed in the body, and which should have been eliminated as 

 soon as formed by the extirpated or diseased organs. So essential, 

 indeed, to animal life, is this excretion of nitrogenised matters, 

 resulting from the waste of the nitrogenous tissues, that it may be 

 safely said to be universally present, and to have a kidney, or 

 analogous organ, assigned to it, wherever animals exist having an 

 arrangement of different and mutually dependent parts, on such a 

 scale as to render the direct expulsion of the waste materials at 

 the points where they are formed impossible. 



The principal excretions may be thus enumerated: 1. Carbonic 

 acid gas, formed by the action of the oxygen upon carbon, and 

 separated by the lungs; its accumulation in the blood is very 

 rapidly fatal to life. 2. Urea, uric acid (in herbivorous animals 

 hippuric acid), kreatine, and Jcreatinine : all nitrogenized principles 

 of definite composition, resulting principally from the waste of the 

 tissues, and eliminated by the kidneys ; their retention in the blood 

 is soon, but less rapidly, fatal. 3. Various saline matters, separated 

 by the kidneys and skin. 4. Lactic add, principally by the skin. 



