NUTRITIVE DECOMPOSITION. 87 



puric acid, as well as many earthy carbonates. In many 

 birds, and in most reptiles, the urine is composed mostly of 

 uric acid ; whilst in frogs and tortoises (turtles and tortoises) 

 it contains urea and albumen. Its composition in fishes 

 appears to be the same ; but in insects we again find uric acid. 

 Disease affects its composition in man. 



164. The extreme rapidity with which various drinks, 

 medicated or simple, pass from the stomach to the bladder, 

 and so escape externally by the urethra, is well known ; yet 

 it is certain that these fluids have first mingled with the blood 

 and been by it carried to the kidneys. 



165. A variety of circumstances, unnecessary to dwell 

 on, influence the activity of the secretion and modify its 

 character. The liquids, and especially water, taken into the 

 stomach escape either by pulmonary or cutaneous exhalation, 

 or by the kidneys as urine. With heat, the cutaneous exha- 

 lation is increased ; by cold, the urinary. 



The amount of solid substances secreted by the kidneys 

 depends greatly on the abundance and nature of the food. 

 It is diminished during a prolonged fast ; and is rich in its 

 solid contents in proportion to the animalization of the food 

 employed. 



166. Various deposits are found in the urinary passages. 

 These are called gravel and urinary calculi, or concretions. 

 The former is almost always formed of uric acid. The depo- 

 sits commence usually, if not always, in the kidneys. Uri- 

 nary concretions also usually form in the kidneys, but 

 descending from these into the bladder, increase, by deposits 

 on their surface, to a size endangering life, and requiring for 

 their removal a surgical operation. 



OF ASSIMILATION AND NUTEITIVE DECOMPOSITION. 



167. Assimilation. The substances introduced into 

 the animal economy are there employed in two ways. They 

 serve for the formation of the different parts of the body 

 itself, or to support the respiratory combustion which con- 

 stantly exists in the interior of every animal so long as life 

 exists. 



But neither animals nor plants can of themselves form any 

 of the simple substances of which their bodies are composed, 

 and therefore the foreign matters thus introduced must con- 

 tain all their elements. 



