86 CHEMICAL COMPONENTS OP THE HUMAN BODY. 



still rarer to discover it in the urine which has been rendered alkaline by the 

 general conditions of the system. As a general rule, it may be stated, that no 

 conclusions can be drawn respecting the amount of uric acid in the urine, from 

 the formation of a sediment, either of this substance or of one of its salts; since 

 the deposition of this sediment will be determined by a number of conditions which 

 affect its solubility, independently of those which occasion variations in the ab- 

 solute quantity produced. Among these, not the least important is the amount 

 of the watery portion of the secretion; since a diminution of this may occasion 

 a precipitation of urate of soda or urate of ammonia on its cooling, although these 

 salts were dissolved in it at the temperature of the body. So, again, the pre- 

 sence of lactic acid may occasion a precipitation of uric acid, although the latter 

 has not been formed in excess. The peculiar conditions of the system which 

 give rise to these deposits, will have to be considered hereafter (CHAPTER xii., 

 SECT. 3). 



57. Tracing back the formation of Uric acid, as we have attempted to do in the 

 case of Urea, we find ample evidence, in the first place, that it is not generated 

 in the act of secretion, but that it pre-exists in the blood ; as it has been de- 

 tected there, not merely in diseased states of the system, in which either its 

 elimination is checked, or its production is increased, or both conditions concur ; 

 but even, though in very minute quantity, in healthy blood. 1 Urate of soda 

 is found in abundance in gouty concretions and tophaceous deposits; and it 

 has been observed by Dr. Gr. Bird as a sort of efflorescence on the surface of 

 the limbs of patients suffering under rheumatic gout. 2 It cannot be doubted 

 that Uric acid is formed, either directly or indirectly, by the metamorphosis 

 of the protein-compounds, whether this be consequent upon the disintegration 

 of the living tissues, or upon the decomposition of superfluous alimentary 

 materials; and we have seen that there is adequate evidence that it may be con- 

 verted into urea in the living body, as in the laboratory of the Chemist, whilst 

 there is no corresponding evidence that urea can be converted into uric acid. 

 When it is borne in mind, also, that uric acid is by far the more general of these 

 two substances urea being almost peculiar to the Mammalian class which is 

 remarkable for the fluidity of its urine, whilst uric acid, in combination with 

 soda and ammonia, is the characteristic constituent of the semi-solid urine of 

 the oviparous Yertebrata, as well as of that of many Invertebrate animals 3 

 there seems a strong probability that uric acid is one of the first products (if 

 not actually the first) of that metamorphosis, and that urea is subsequently 

 generated from it during its passage through the circulation. That of the entire 

 amount of uric acid generated in the system, a part is directly derived from the 

 food, when this contains a superfluity of azotized compounds, appears from the 

 experiments of Lehmann, who found that whilst he voided 11.24 grains of uric 

 acid in twenty-four hours, whilst living upon a diet entirely unazotized this 

 quantity, therefore, representing that which results from the "waste" of the 

 tissues alone he disengaged 15.7 grains when living upon a vegetable diet, 

 18.17 grains upon a mixed animal and vegetable diet, and 22.64 grains (or 

 rather more than double the first amount) when his diet was exclusively animal. 



1 See Dr. Garrod's Observations on certain pathological conditions of the Blood in Gout, 

 Rheumatism, and Bright's Disease, in " Medico-Chirurg. Trans." vol. xxxi. 



2 Op. cit. p. 117, Am. Ed. 



A substance termed Guanine, which has weak basic properties, and whose formula is 

 IOC, 6H, 5N, 20, has been discovered by linger in guano, which is the mingled urinary 

 and fecal excrement of sea-fowls ; it has been recently discovered also in the excrements 

 of spiders, by Will and Gorup-Besanez ; and they think it probable that a substance which 

 they find in the urinary organs of the river craw-fish and of the fresh-water mussel, is 

 identical with this. Dr. J. Davy believed that the urinary secretion of scorpions and spi- 

 ders consists for the most part of xanthine or " uric oxide," (| 66 ;) but the substance which 

 he discovered was more probably guanine. 



