EXCREMENTITIOUS SUBSTANCES. 83 



in the urine ; and the question arises, whether Urea is one of the immediate 

 products of this metamorphosis, or whether some other compound is first gene- 

 rated, which is converted into urea after its absorption into the circulation. 

 Now although it is impossible in the present state of our knowledge to answer 

 this question in either way with any degree of certainty, yet there is certainly 

 a strong probability that the second of these views is the correct one ; for, in 

 the first place, no trace of urea has been found in the " juice of flesh ;" whilst, 

 secondly, this juice contains a considerable amount of the substance (creatine), 

 from which urea may be generated artificially ( 60). That uric acid also may 

 be converted into urea in the circulating blood, seems to have been distinctly 

 proved by the experiments to be presently cited ( 55). It appears most pro- 

 bable, therefore, that the products of the disintegration of muscle are received 

 into the blood, not in the form of urea itself, but in that of creatine or of some 

 other compound, which is capable of being resolved into urea by further 

 metamorphosis ; and that the immediate source of urea, therefore, in the latter 

 case as in the former, is one of the constituents of the circulating blood. 



54. Next in importance to Urea, and taking its place as the predominating 

 component of the urinary excretion of most animals below Mammalia, is Uric Acid. 

 The small proportion, however, in which this substance normally presents itself 

 in the urine of Man its usual amount being less than 1 part in 1000 of Urine, 

 whilst that of Urea is from about 25 to 32 parts in 1000 might seem to ren- 

 der it unnecessary to enter at length into its chemical peculiarities or its physi- 

 ological relations. But although in the healthy system it may seem to perform 

 a subordinate part, there can be no doubt whatever that in various disordered 

 states of the body an augmented production of uric acid, or a deficient meta- 

 morphosis or elimination of it, or both conditions combined, exercise a very 

 important influence, and become the sources of other perversions; and it is 

 therefore a matter of great consequence to trace out, so far as is possible, the 

 entire history of this substance, both chemical and physiological. Pure uric 

 acid occurs either in a glistening white powder, or in very minute scales, whose 

 form, when they are examined by the microscope, is found to be distinctly 

 crystalline, although the shape of the crystals is liable to considerable variation. 

 As it is by the microscope, rather than by any chemical tests, that the presence 

 of uric acid is most readily detected, and as its Protean forms of crystallization, 

 although easily reduced by the crystallographer to a determinate system, are 

 liable to perplex those who have not the advantage of his scientific knowledge, 

 it will be advantageous here to cite the description of them given by Prof. 

 Lehmann. "Uric acid, when it gradually and spontaneously separates from 

 urine, appears in most cases in the whetstone form, that is, it forms flat tablets, 

 which resemble sections made with the double knife through strongly bi-convex 

 lenses, or rhombic tablets whose obtuse angles have been rounded. As the 

 urinary pigment adheres very tenaciously to the uric acid, it is only rarely that 

 these crystals are devoid of color, and if we see a crystal presenting an extra- 

 ordinary form and of a yellow color, the probability is that it is a crystal of 

 uric acid. On artificially separating uric acid from its salts, it often appears in 

 perfect rhombic tablets, and even oftener in six-sided plates resembling those of 

 cystine ; when uric acid crystallizes very slowly, it forms elongated rectangular 

 tablets or parallelepipeds, or rectangular four-sided prisms, with horizontal ter- 

 minal planes; the latter are often grouped together in clusters; we have also 

 barrel-shaped or cylindrical prisms, which are composed of the more rarely- 

 occurring elliptic tablets; and finally saw-like or toothed crystals, and many 

 derivatives of these forms. If we cannot decide with certainty regarding the 

 presence of uric acid from the form of a crystal, we must dissolve it in potash, 

 place it under the microscope, and add a minute drop of acetic acid; we shall 



