414 OF SECRETION. 



kreatinine is gradually resolved into urea; and thus it would seem as if 

 they hold an intermediate position between the components of the orga- 

 nized tissues and the urea which may be considered as the ultimate pro- 

 duct of their metamorphosis within the body. The peculiar crystalline 

 compound just referred to, as having been formerly supposed to be lac- 

 tate of zinc, has been shown to be really formed by a combination of 

 zinc with a compound of kreatine and kreatinine. 



736. Of the substances ranked under the head of Extractive Matters, 

 little is definitely known ; it appears, however, from recent researches, 

 that they are peculiarly rich in carbon, and that they are liable to be 

 greatly augmented, either by an excess of non-azotized matter in the 

 food, or by any impediment to the action of the liver or lungs. A 

 yellow extractive has been separated by Scherer, which he regards as 

 proceeding from the final metamorphosis of the haematin of the blood ; 

 and this seems nearly related to the purpurine, which sometimes gives 

 a deep colour to the sediment of urate of ammonia, and which is pecu- 

 liarly liable to appear when the functional activity of the liver is below 

 par. The ordinary colouring-matter of bile often presents itself in the 

 urine in cases of jaundice : and, as Dr. Golding Bird has pointed out, 

 there is a close similarity in composition between these three coloured 

 compounds, indicating a derivation from the same source. A sulphur 

 extractive has also been obtained from the urine : in which (as in the 

 bile) there is a considerable proportion of free sulphur. 



737. The Urine also contains a considerable amount of Saline matter, 

 of which the acids as well as the bases are derived from the mineral 

 kingdom; and the excretion of them, after they have served their pur- 

 pose in the economy, appears to be one of the chief functions of the 

 Kidney. Of these a part may find their way directly into the urine 

 from the serum of the blood, when its water is being filtered off (so to 

 speak) through the walls of the Malpighian capillaries ; for although, 

 from the peculiar properties of animal membranes ( 196), the albumi- 

 nous constituents of the serum are held back, the saline matter, which is 

 in a state of perfect solution, must pass with the water. This is pro- 

 bably the chief source of the large quantity of the muriates of soda and 

 ammonia contained in the urine. But the Urinary secretion seems to be 

 specially destined to eliminate the saline compounds, which are formed 

 by the acidification of the Sulphur and Phosphorus, taken in with the 

 proteine-cornpounds as food. These substances are united with Oxygen 

 in the system, and are thus converted into Sulphuric and Phosphoric 

 acids ; which acids unite with alkaline bases, that were ingested in 

 combination with Citric, Tartaric, Oxalic, and other organic acids; 

 the latter undergoing decomposition within the system, and leaving 

 the bases ready to unite with others. Such weakly-combined bases 

 abound in the food of Herbivorous animals ; and their urine is almost 

 invariably alkaline, the quantity of the Sulphuric and Phosphoric acids 

 generated in the system not being sufficient to neutralize it. On the 

 other hand, they are nearly absent in the food of the Carnivora ; and 

 their urine is therefore almost invariably acid, from the want of neutra- 

 lization of the Sulphuric and Phosphoric acids. 



738. The Alkaline Sulphates, whether taken-in as such, or formed in 



