SUGARS. 



69 



Urea normally constitutes about 13 parts in 1,000 of urine. 

 (Bird.) The amount is increased by a nitrogenized diet, and by 

 muscular exercises. 



Origin. It is decided that urea is formed in the blood, and it is 

 doubtless formed from oreatine, uric acid, and probably inosic acid 

 also, by the action of the alkalies, and of free oxygen. (Lehmann.) 

 And since creatine is produced by the waste of muscular tissue, 

 strong muscular exercise increases the urea in the urine. Thus, 

 also, in delirium tremens, and all states attended by intense muscu- 

 lar actions (convulsions, &c.), a similar increase occurs. 



But urea probably also results from the decomposition of any 

 tissue containing nitrogen, and not from that of the muscles alone. 

 Moreover, if an excess of nitrogenized food is absorbed into the 

 blood, it is excreted in the form of urea, this substance being the 

 last and lowest step in the descending scale of the metamorphosis 

 of the tissues, while the lactic, uric, and oxalic acids, creatine, and 

 creatinine constitute the preceding grades. The idea of a <( urea 

 diathesis" is thus seen to be un- 

 tenable. Fig- 38. 



4. Cystine. (C 6 H 6 N0 4 S 2 .) 



Cystine occurs in colorless, 

 transparent, hexagonal plates 

 and prisms (Fig. 38), and only 

 in the urine, and in pathological 

 states. It is richer in sulphur (it 

 constituting 25 per cent.) than 

 any other organic substance, 

 except taurine. It sometimes 

 forms calculi. 



Nothing is known of the con- 

 ditions of the formation of cys- Cystine from urinary calculuS) recr78tallized from 

 tine in the organism. ammonia. 



THIRD DIVISION. 

 Sugars, or Neutral Non-nitrogenized Immediate Principles. 



But two kinds of sugar are found in animals sugar of the liver, 

 and sugar of milk. In vegetables there are several kinds; and 

 grape sugar, or glucose, has the same composition as hepatic sugar. 



