NUTEITIVE DECOMPOSITION. 83 



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 descend- 

 ing 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 NUTBITIVE 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 constantly 

 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. 



The primary materials of the organism are carbon, nitrogen 

 or azote, hydrogen, and oxygen ; but sulphur, phosphorus, 

 lime, and other simple bodies, are also required ; it is essen- 

 tial, then, that such bodies should be introduced from without. 

 But animals do not possess the faculty of determining the 

 combination of these various chemical elements, so as to give 

 rise to the various compound principles of which the organism 

 is formed ; or, in other words, these elements must be already 

 combined. Thus, it is not by introducing azote, hydrogen, 

 carbon, &c., into the body that an animal can satisfy the 

 G 2 



