646 OF SECRETION. 



inferences, that the statements concerning the amount of Lactic acid and the 

 Lactates, must be considered as invalidated by the discoveries of Liebig al- 

 ready referred to ( 846). The most unequivocal facts determined by Dr. 

 Lehmann's inquiries, are those which relate to the influence of Diet on the 

 amount of Urea excreted. The experiments upon a purely non-azotized 

 diet were not continued long enough for a satisfactory result to be obtained ; 

 but it is evident that, so long as the ingesta contain no azote, the whole of 

 that element in the Urine must be attributed to the disintegration or waste of 

 the tissues, and may be fairly taken as a measure of its amount. 



851. The fact of the pre-existence of the chief constituents of Urine in the 

 Blood, is important as explaining the facility with which the secreting function 

 appears to be transferred to other membranes, in some of the cases in which 

 the Kidney does not perform its function. Doubtless there has been much 

 error on this subject, arising out of deceptions practised by impostors ; but a 

 sufficient number of indubitably genuine cases are on record, to put it beyond 

 doubt that such transferences have taken place, urinous fluid being secreted 

 from the stomach, mammae, umbilicus, nose, &c.* On the other hand, the 

 Kidney may serve as the channel for the elimination of substances which 

 are usually drawn off by other organs. Thus, when the secreting action of 

 the Liver has been gradually impaired by structural disease, the Kidneys ap- 

 pear to have performed its function, in separating some (at least) of the ele- 

 ments of Bile. And a case has recently been mentioned, in which the urine 

 of a parturient female, who did not suckle her infant, was found to contain a 

 considerable amount of Butyric acid, during several days. The elimination 

 of Kiesteine by the Kidney during pregnancy will be presently noticed ( 859). 



852. The facility with which substances taken into the current of the 

 Circulation pass into the Urinary secretion, varies extremely ; and no general 

 law can be stated in regard to it. It appears from Wb'hler's elaborate re- 

 searches on this subject, that the salts which are most readily excreted are 

 those which excite the action of the kidneys.t The rapidity with which 

 absorption and elimination take place is often extremely remarkable; Prus- 

 siate of Potash having been detected in the urine, within two minutes after it 

 had been introduced into the stomach. The variations in this respect would 

 appear to depend chiefly on the degree of concentration of the saline solution, 

 which will affect the rapidity of its absorption, according to the laws of En- 

 dosmose; its reception into the blood being more rapid, in proportion as its 

 density is lower, in comparison with that of the circulating fluid. Pure water, 

 or water containing but a small admixture of saline matter, is readily absorbed 

 into the blood-vessels of the Intestinal villi ; but it is as readily drawn off 

 through the Kidneys (by the agency, as it would seem, of the Malpighian 

 bodies, 840) ; and consequently a large amount may be ingested in a short 

 time. But if the water contain an amount of saline matter equal to that of 

 the Serum, no absorption of it takes place; it remains in the intestinal tube; 

 and it is voided by the rectum. Further, if the quantity of saline matter in 

 the solution be greater than that of the Serum, not only will no absorption 

 take place, but there will be an endosmose of the water of the blood towards 

 the solution; so that a large quantity of fluid is discharged by the Intestinal 

 canal. This simple explanation, first offered by Liebig,:): accounts well for 

 the diuretic effect of most weak saline solutions, and the purgative qualities of 

 stronger ones. For the transit of the peculiar principles of Vegetables, how- 

 ever, it appears that from one to two hours are usually required. The effect 



* For a scientific explanation of this fact, see Princ. of Gen. and Comp. Phys., 539. 



+ See Miiller's Physiology, p. 589. 



4! Chemistry applied to Agriculture and Physiology, Part ii. 



