GENERAL CONSIDERATIONS. 379 



This fluid does not differ much in its composition from ordinary milk. The propor- 

 tion of butter is much less, but the amount of sugar is greater, and the quantity of case- 

 ine is nearly the same. 



Of the other fluids which are enumerated in the list of secretions, the saliva, gastric 

 juice, pancreatic juice, and the intestinal fluids have already been considered in connec- 

 tion with digestion. The physiology of the lachrymal secretion will be taken up in con- 

 nection with the eye, and the bile will be treated of fully under the head of excretion. 



CHAPTER XII. 



EXCRETION- BY THE SKIN AND KIDNEYS. 



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

 anatomy of the nails and hairs Sudden blanching of the hair Uses of the hairs Perspiration 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 Distribution of blood-vessels in the kidneys 

 Lymphatics and nerves of the kidneys Mechanism of the production and discharge of urine Formation 

 of the excrementitious constituents of the urine in the tissues, absorption of these principles by the blood, 

 and separation of them from the blood by the kidneys Effects of removal of both kidneys from a living animal 

 Effects of tying the ureters in a living animal Extirpation of one kidney Influence of blood-pressure, the 

 nervous system, etc., upon the secretion of urine Alternation in the action of the kidneys upon the two sides- 

 Changes in the composition of the blood in passing through the kidneys Physiological anatomy of the urinary 

 passages Mechanism of the discharge of urine Properties and composition of the urine General physical prop- 

 erties of the urine Quantity, specific gravity, and reaction of the urine Composition of the urine Gases of 

 the urine Variations in the composition of the urine Variations produced by food Urina potus, urina cibi, 

 and urina sanguinis Influence of muscular exercise upon the urine Influence of mental exertion. 



IN entering upon the study of the elimination of effete matters, it is necessary to 

 appreciate fully the broad distinctions between the secretions proper and the excretions, 

 in their composition, the mechanism of their production, and their destination. These 

 considerations are again referred to, for the reason that they have not ordinarily received 

 that attention in works upon physiology which their importance seems to demand. The 

 mechanism of excretion is inseparably connected with the function of nutrition, and it 

 forms one of the great starting-points in the study of all the modifications of nutrition in 

 diseased conditions. 



Taking the urine as the type of the excrementitious fluids, it is found to contain none 

 of those principles included in the class of non-crystallizable, organic nitrogenized mat- 

 ters, but is composed entirely of crystallizable matters, simply held in solution in water. 

 The character of these principles depends upon the constitution of the blood and the gen- 

 eral condition of nutrition, and not upon any formative action in the glands. The principles 

 themselves represent the ultimate physiological changes of certain constituent parts of the 

 living 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 inorganic matters are found 

 in the excrementitious fluids, are discharged with the products of excretion, and are thus 

 associated with the organic principles of the economy in their physiological destruction, 

 as well as in their deposition in the tissues. Coagulable organic matters, or albuminoid 

 principles, never exist in the excrementitious fluids under normal conditions ; except as 

 the products of other glands may become accidentally or constantly mixed with the 

 excrementitious fluids proper. The same remark applies to the non-nitrogenized matters 

 (sugars and fats), which, whether formed in the organism or taken as food, are consumed 

 as such in the process of nutrition. The production of the excretions is constant, being 

 subject only to certain modifications in activity, which are dependent upon varying con- 

 ditions of the system. All of the elements of excretion preexist in the blood, either in 

 the precise condition in which they are discharged or in some slightly-modified form. 



