OF EXCRETION. 213 



CHAPTER XII. 



OF EXCRETION. 



THE URINE, MILK, AND CUTANEOUS EXCRETIONS. 



Secretion and Excretion. 



Of the Kidney: its Structure and Functions. The Malpighian Circulation. The Urine: its In- 

 gredients, their Variations and Sources. Abnormal Substances in it. The Water and Salts 

 exude by Filtration. The Cells remove unoxidized Bodies. Manner of Removal of the Liquid 

 from the Malpighian Sac. 



Of the Mammary Gland: its Structure. Colostrum and Milk. Ingredients of Milk and their 

 Variations. Influence of Diet. Inquiry into the Origin of the Ingredients of the Milk, its Fat, 

 Casein, Salts, Sugar. Manner of Action of the Gland by Strainage. 



Of the Skin. Structure of its Epiderma and Derma. Sudoriparous and Sebaceous Glands. 

 Nails. Hair. Ingredients of Perspiration. Exhalation: its Amount. Causes of the Vari- 

 able Action of the Skin. Its Double Action. Absorption by the Skin. General Summary of 

 the Cutaneous Functions. + 



THE function of secretion is very commonly treated of by physiolo- 

 gists under two divisions, secretion and excretion. The Di s t i nct ; on be 

 former refers to the separation from the blood of those fluids tween secretion 

 which are required for the uses of the body, and which are 

 therefore still retained ; the latter, to those which are effete, and to be 

 cast out as excrementitious matter. Of secretions, the saliva or the pan- 

 creatic juice may be taken as examples ; of excretions, the urine. 4 



But this subdivision is only one of convenience, and has no natural 

 foundation. The so-called secretions are, in many instances, far from 

 being more highly elaborated bodies ; in reality, they are often on their 

 descending career. And among excretions, if milk be enumerated, as it 

 ought to be, since it is a dismissed product of the system preparing it, 

 we have, instead of an excrementitious, a pre-eminently nutritive body. 



Nevertheless, since this manner of considering the subject offers con- 

 siderable conveniences, I have resorted to it for the preceding and pres- 

 ent chapters. In this I shall accordingly treat of the urine, the milk, 

 and the products removed by the skin. 



OF THE KIDNEYS. 



The products of waste arising from oxidation in the functional activity 

 of the system, and which are of a non-gaseous kind, the use- , 



J ' . Physical func- 



less materials, saline or otherwise, which have been absorb- tion ofthekid- 

 ed in the digestive tract, and carried into the circulation, ne7 ' 

 must be removed. Gaseous substances and vapors may pass away 

 through the lungs, but solid material must be excreted in a state of so- 



