CHEMISTRY OF DIGESTION AND NUTRITION. 273 



the splanchnic nerves, which carry also inhibitory fibres whose stimulation pro- 

 duces a dilatation of the spleen. 



The chemical composition of the spleen is complicated but suggestive. Its 

 mineral constituents are characterized by a large percentage of iron, which 

 seems to be present as an organic compound of some kind. Analysis shows 

 also the presence of a number of fatty acids, fats, cholesterin, and, what is 

 perhaps more noteworthy, a number of nitrogenous extractives such as 

 xanthin, hypoxanthin, adenin, guanin, and uric acid. The presence of 

 these bodies seems to indicate that active metabolic changes of some kind occur 

 in the spleen. As to the theories of the splenic functions, the following may be 

 mentioned : (1) The spleen has been supposed to give rise to new red corpuscles. 

 This it undoubtedly does during fetal life and shortly after birth, and in some 

 animals throughout life, but there is no reliable evidence that the function is 

 retained in adult life in man or in most of the mammals. (2) It has been 

 supposed to be an organ for the destruction of red corpuscles. This view is 

 founded partly on very unsatisfactory microscopic evidence according to which 

 certain large amoeboid cells in the spleen ingest and destroy the old red corpus- 

 cles, and partly upon the fact that the spleen-tissue seems to be rich in an iron- 

 containing compound. This theory cannot be considered at present as anything 

 more than a suggestion. (3) It has been suggested that uric acid is produced 

 in the spleen. This substance is found in the spleen, as stated above, and it has 

 been shown recently by Horbacewsky that the spleen contains a substance 

 from which uric acid or xanthin may readily be formed ; but further investiga- 

 tion has shown that the same substance is found in lymphoid tissue generally. 

 If, therefore, uric acid is produced in the spleen, it is a function of the large 

 amount of lymphoid tissue contained in it, and a function which it shares with 

 similar tissues in the rest of the body. The lymphoid tissue of the spleen must 

 also possess the property of producing lymphocytes, since, according to the gen- 

 eral view, these corpuscles are formed in lymphoid tissue generally wherever 

 the so-called " germ-centres " occur. (4) Lastly, a theory has been supported 

 by Schiff and Herzen, according to which the spleen produces something (an 

 enzyme) which, when carried in the blood to the pancreas, acts upon the tryp- 

 sinogen contained in this gland, converting it into trypsin. The experimental 

 evidence upon which this view rests has not been confirmed by other observers. 



G. THE KIDNEY AND THE SKIN AS EXCRETORY ORGANS. 

 The secretion of the kidneys is the urine. The means by which this secre- 

 tion is produced, its relations to the histological structure of the kidney, and 

 its connections with the blood- and nerve-supply of that organ will be found 

 described in the section on Secretion. In this section will be discussed only 

 the chemical composition of urine, and especially the physiological significance 

 of its different constituents. The urine of man is a yellowish liquid varying 

 greatly in depth of color. It has an average specific gravity of 1020, and an 

 acid reaction. The acid reaction is not due to a free acid, but to an acid salt, 

 the acid phosphate of sodium (NaH 2 PO 4 ). Under certain normal conditions 



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