DIFFERENCES BETWEEN SECEETIONS AND EXCRETIONS. 341 



mucous membranes, the gastric juice, the intestinal juice, the sebaceous 

 fluids, the milk or the lachrymal fluid. 



The centres for the salivary secretions are in the medulla oblongata, near 

 the points of origin of the facial and glosso-pharyngeal nerves. The centre 

 for the pancreatic secretion is also in the medulla oblongata. The centres 

 which act upon the liver and upon certain excretions will be treated of in 

 connection with the physiology of the liver, kidneys and skin. 



CHAPTER XII. 



EXCRETION BY THE SKIN AND KIDNEYS. 



Differences between the secretions proper and the excretions Physiological anatomy of the skin Physio- 

 logical anatomy of the nails Physiological anatomy of the hairs Sudden blanching of the hair Per- 

 spiration Sudoriparous glands Mechanism of the secretion of sweat Properties and composition of 

 the sweat Peculiarities of the sweat in certain parts Physiological anatomy of the kidneys Mechan- 

 ism of the production and discharge of urine Influence of blood-pressure, the nervous system etc., 

 upon the secretion of urine Physiological anatomy of the urinary passages Mechanism of the dis- 

 charge of urine Properties and composition of the urine Influence of ingesta upon the composition 

 of the urine and upon the elimination of nitrogen Influence of muscular exercise upon the elimination 

 of nitrogen Water regarded as a product of excretion Variations in the composition of the urine. 



IN entering upon the study of the elimination of effete matters, it is ne- 

 cessary to appreciate fully the distinctions between the secretions proper and 

 the excretions, in their composition, the mechanism of their production, and 

 their destination. The urine may be taken as the type of the excrementi- 

 tious fluids. None of its normal constituents belong to the class of non-crys- 

 tallizable, organic nitrogenized matters, but it is composed entirely of crys- 

 tallizable matters, simply held in solution in water. The solid constituents 

 of the urine represent the ultimate physiological changes of certain parts of 

 the organism, and they are in such a condition that they are of no farther 

 use in the economy and are simply discharged from the body. Certain in- 

 organic matters are found in the excrementitious fluids, are discharged with 

 the products of excretion, and are thus associated with the organic constitu- 

 ents of the body in their physiological changes as well as in their deposition 

 in the tissues. Coagulable organic matters, or albuminoids, never exist in 

 the excrementitious fluids under normal conditions ; except as the products 

 of other glands may become accidentally or constantly mixed with the ex- 

 crementitious fluids proper. The same remark applies to the non-nitrogen- 

 ized matters, sugars and fats, which, whether formed in the organism or 

 taken as food, are consumed in the organism. The production of the excre- 

 tions is constant, being subject only to certain modifications in activity, 

 which are dependent upon varying conditions of the system. All of the 

 elements of excretion pre-exist in the blood, either in the condition in 

 which they are discharged or in some slightly modified form. 



The urine is a purely excrementitious fluid. The perspiration and the 



