EXCREMENTITIOUS SUBSTANCES. 81 



struction of living tissues. And of all of them it may be said, that their accu- 

 mulation in the Blood is extremely deleterious ; so that, if they should be gene- 

 rated faster than the excretory organs can remove them, a serious derangement 

 of the vital economy is produced. These peculiarities separate the proper 

 excrementitious substances from all those which are introduced into the body 

 as food, and which serve either for the genesis of tissue, or for the supply of the 

 combustive process ; but, as already remarked, the two groups are connected by 

 intermediate links cholesterine, though a normal element of the blood, being 

 a component of the biliary excretion whilst lactic acid, though a most import- 

 ant ingredient in the gastric juice, is an essentially excrementitious product of 

 the metamorphosis of the saccharine compounds, whose constant and complete 

 elimination from the blood seems to be most carefully provided for. Like the 

 histogenetic substances, the proper Excrementitious matters may be divided 

 into two principal groups, in one of which nitrogen is the predominating and 

 characteristic ingredient, whilst in the other we find carbo-hydrogen in excess ; 

 and these two groups form the chief components of the Urinary and the Biliary 

 excretions respectively. 



52. Of the first of these groups, the substance of by far the most importance 

 in the Human body, is Urea ; and this, although not possessing an alkaline 

 reaction, is regarded as basic, since it has the power of forming saline compounds 

 with acids. Urea is obtained from the fluids containing it, in a crystalline 

 form : but the shape of its crystals varies according as it separates rapidly or 

 gradually ; these being white, silky, glistening needles in the former case ; and 

 flat, colorless, four-sided prisms, terminated by one or two oblique surfaces, in 

 the latter. Urea is devoid of smell, has a saltish cooling taste, and is unaffected 

 by exposure to the atmosphere. It dissolves readily in its weight of cold water, 

 and in hot water in every proportion ; it is also soluble in 4 or 5 parts of cold, 

 and in 2 parts of warm alcohol ] and it is not precipitated from its solutions by 

 metallic salts, tannic acid, or any other reagent. It has been stated to combine 

 as a base with a considerable number of acids ; but the only salts which it is 

 certainly known to form, are the nitrate, oxalate, and hydrochlorate , and it is 

 by the use of either nitric or oxalic acid, that urea is most readily obtained from 

 its solutions, since the salts which it forms with them are far less soluble in 

 water than is urea itself. Urea unites also, however, with many salts, some of 

 which hold it in such firm combination, that it cannot be thus separated from 

 them. The composition of Urea is extremely simple in comparison with that 

 of the azotized histogenetic substances, its formula being 2 Carbon, 4 Hydrogen, 

 2 Nitrogen, and 2 Oxygen ; but Chemists are not yet agreed upon the mode in 

 which these atoms are united. The number of the combining equivalents of the 

 respective elements is the same as that which exists in the Cyanate of Ammonia ; 

 and it was long since ascertained by Wohler, that this salt may be converted 

 into (basic) urea by the rearrangement of its atoms, without any alteration in 

 its ultimate composition. This artificial production of urea may be considered 

 as one of the most interesting discoveries ever made in organic chemistry. When 

 organic matters, either putrefying or capable of undergoing putrefaction, are 

 mixed with an aqueous solution of Urea, the latter is soon converted, with the 

 addition of two equivalents of water, into Carbonic acid and Ammonia ; thus 

 restoring to the atmosphere, in their original state of combination, the compounds 

 at whose expense the Plant first generated the organic constituents of the Animal 

 body. This change takes place in the Urine very soon after its expulsion from 

 the body ; the mucus of the bladder acting as the ferment required. 1 



1 See MM. Dumas and Boussingault " On the Chemical and Physiological Balance of 

 Organic Nature," 3d edit, (transl.), p. 41 ; and the Author's "Principles of Physiology, 

 General and Comparative," p. 186, Am. Ed. 



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