THE SUBSTANCES OXIDIZED. 841 



the partially effete matters of the blood, and, lastly, with the materials 

 of the respiratory food. Hence, the carbonic acid, which is one of the 

 ultimate products of these changes, is derived partly from the disinte- 

 grated substance of the tissues, especially the nervous and muscular 

 tissues, and partly from substances merely assimilated into the blood 

 from the food. The oxidation of the respiratory food appears to take 

 place in the blood itself, partly in the lungs, but chiefly in the arterial 

 blood-current, and, during muscular action, largely in the capillaries 

 of the muscles. The oxidation of the substance of the tissues may 

 occur, partly, in the tissues themselves, outside the walls of the sys- 

 temic capillaries, but partly, also, in the blood itself, into which certain 

 intermediate effete products of disintegration may enter, and there 

 undergo further oxidation, after the manner of the respiratory food. 



The second point above suggested for consideration, viz., the nature 

 of the substances immediately oxidized, must remain at present in ob- 

 scurity. That they contain carbon is, however, certain. Some of this 

 united with oxygen forms the carbonic acid given off in respiration. 

 They also contain hydrogen, frequently in excess of the quantity of 

 oxygen atomically present in them ; and then, by oxidation of this 

 hydrogen, water must be formed in the system. But some carbon and 

 some hydrogen escape perfect oxidation, appearing, combined with 

 nitrogen and a little oxygen, in the urea, uric, and hippuric acids, and 

 other nitrogenous excretory compounds. The sulphur contained in 

 the albuminoid bodies, and the phosphorus present more especially in 

 the phosphuretted fats of the red blood corpuscles and in the gray 

 nervous substance, are likewise oxidized, at the cost of the oxygen 

 taken in by the respiratory process, so as to form sulphuric and 

 phosphoric acids, which appear in combination with alkalies or earthy 

 matters, in the urine. The sulphur may be partly traced in the 

 intermediate formation of taurin in the bile. With regard to the 

 nitrogen contained in the so-called nitrogenous tissues, it is, as 

 already mentioned, almost entirely accounted for, by the urea and 

 uric acid passing, probably, through intermediate chemical forms, 

 such as creatin, creatinin, sarcin, glycin, allantoin, and others. 

 Minute quantities appear to escape from the blood, as ammonia. The 

 nitrogen exhaled from the lungs, and the small loss of that substance 

 from the epidermis and the intestinal excreta, are not metamorphic, 

 the former being derived from the air swallowed with the ingesta and 

 the saliva, and the latter being contained in non-metamorphosed 

 organized matter. Cholesterin is also a scarcely oxidized excretory 

 hydro-carbon. 



The great object, therefore, of the respiratory function is to intro- 

 duce oxygen into the living animal economy; this oxygen, by giving 

 rise to numerous and incessant chemical changes, stimulates the animal 

 tissues, combines with their substance and with the products of their 

 disintegration, and ultimately converts them, either into crystalloid 

 products, which can be readily excreted from the kidneys or skin, or into 

 a gas very soluble in water and in the blood which can be readily 

 displaced from the latter fluid, in the lungs or in other respiratory 

 organs, by the stronger affinity of oxygen itself for a certain constitu- 



