SECRETION AND EXCRETION COMPARED. 753 



the soda salts of the biliary acids, and the lactin or sugar of the milk, 

 or crystalloid, such as the hydrochloric, acid, and other acids in the 

 gastric juice. All these would freely dialyze from the blood, or from 

 the secreting cells. As to the modified albuminoid substances, which 

 are colloidal, such as salivin, pepsin, pancreatin, and casein, it is pos- 

 sible that the secreting cells may themselves burst, and yield up their 

 albuminoid contents ; or the secretion of such substances from the 

 blood may present us with examples of the metastasis of colloidal sub- 

 stances from the pectous to the liquid, or from the liquid to the pectous 

 state, as occasion may require. 



In the process of excretion, it is, as already mentioned, the highly 

 diffusible crystalloids alone, which escape from the blood, so that it 

 may more readily be referred to a pure dialysis, the one condition 

 necessary, say, in the secretion of urea, e. g., being a special chemical 

 relation between the dialyzing epithelial cells and the dialyzable urea, 

 which serves to locate the excretion of that substance in the kidney. 



In the formation of living vegetable tissues, crystalloids, such as 

 ammonia, carbonic acid, and water, are converted into colloids, and 

 the further processes of organization, up to the final and highest nu- 

 tritive stage, require various metastases of these colloids. In the 

 downward step of disintegration and disorganization, materials are 

 formed which are to be excreted, and then the crystalloid condition 

 of matter again prevails, as in the urea and uric acid thrown off by the 

 kidneys, which easily pass into ammonia, and the carbonic acid and 

 water of the cutaneous and pulmonary exhalations. 



The general forms of the secreting and excreting glands, and the 

 mode in which those forms may be derived from the involution of a 

 simple secreting membrane, have already been described (p. 64). In 

 all cases, there is invariably found, even in the ultimate ramifications 

 of the gland ducts, a limiting or basement-membrane covered by a 

 stratum of epithelial cells. All glands are, moreover, very vascular, 

 and receive large quantities of blood. The special secretions and ex- 

 cretions are the products or educts of special organs. The most 

 essential modifications of the anatomical gland-elements are those 

 which relate to the epithelial cells. Jn secretion proper, these im- 

 portant elements are frequently dissolved or ruptured, and their con- 

 tents, if not their envelopes, escape as part, perhaps an essential part, 

 of the secretion itself, as in the case of the saliva, pancreatic fluid, 

 gastric juice, and milk, and of the sebaceous and the mucous secre- 

 tions, and also, perhaps, of the bile. But in the case of the lachrymal 

 secretion, and in the excretory processes generally, this is not so ; for 

 the epithelial cells in the ducts of the kidney, the lachrymal gland, 

 and the sweat-glands, and also, it may be added, in the air-cells of the 

 lungs, merely withdraw, as it were, by a special attraction, certain 

 products already pre-existing in the blood, and part with them again, 

 into the ducts or canals, which convey them out from the body, with- 

 out themselves undergoing any necessary dissolution or decay. 



The liver receives a peculiar venous blood, loaded with the products 

 of the venous absorption of the food, and with those which enter the 

 blood of the spleen ; but, with this exception, the cause of the differ- 



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