INORGANIC SUBSTANCES. 105 



80. Although Silica is one of the most important mineral ingredients of 

 many Plants, and is also abundant in some of the lower forms of Animal struc- 

 ture, yet it is present to only a very small extent in the Human organism, the 

 sole tissue of which it seems to be a normal constituent being the Hair. 1 Ac- 

 cording to the analysis of Laer and Grorup-Besanez, about 0.22 per cent, of silica 

 is to be found in the ordinary brown hair of man ; the ashes of which contain 

 nearly 13.9 of this earth. It has been discovered in the blood of Man by Mil- 

 Ion, and in that of the Ox by Weber, in both cases, however, in a quantity not 

 exceeding 0.20 per cent, of the ash ; in the ash of birds' blood, on the other 

 hand, its proportion is about five times as great as might be expected from the 

 larger demand for it in their organisms. Silica has been detected also in the 

 bile and urine, and is obviously carried out of the system through these chan- 

 nels ; what is found in the solid excrements has obviously been derived directly 

 from the food. It is in the seed-coats of many seeds, especially those of 

 the Monocotyledonous division, that the chief source of the silica introduced 

 into the bodies of animals is to be found ; and hence it is that the feathers of 

 granivorous birds present a much larger proportion of silica than any others, 

 and that granivorous quadrupeds are peculiarly subject to intestinal concretions 

 including a large quantity of silica. 



81. Among those inorganic components of the Human body, whose function 

 is rather chemical than mechanical, we may first notice Hydrochloric acid, which 

 may be regarded as an occasional, if not a constant component of the gastric 

 fluid. 3 Both lactic and hydrochloric acids have a powerful action on albuminous 

 substances ; and there is reason to think that the acid reaction and solvent 

 powers of the gastric fluid may be due to either one or the other. In the gas- 

 tric fluid of Man, however, it seems certain (as will be shown, hereafter, CHAP. 

 Vii.) that free hydrochloric acid normally exists, in such a proportion as to 

 render it efficient as the solvent. 



82. Of all the mineral constituents of the Human organism, there is none 

 more important in a chemical point of view than Chloride of Sodium. This 

 substance occurs in nearly every part of the body, both solid and fluid, in close 

 and intimate relation with the organic compounds, whose chemical and physical 

 properties are materially influenced by it : thus Albumen partly owes its solu- 

 bility to this salt, and the differences which it presents in coagulating are in great 

 degree dependent upon the quantity of it that is present; pure Casein, which is 

 otherwise insoluble, is also dissolved by common salt; and if salt be added in 

 increased proportion, it has the power of impeding the coagulation of the Fibrin 

 of the blood. Moreover, this substance js not only uniformly present, but exists 

 in nearly definite and constant proportions, in the several tissues and fluids; 

 and the existence of a provision for the limitation of the quantity retained in 

 the system renders these proportions but little liable to be affected, in the way 

 of excess at least, by the quantity of salt which the food may contain. Thus 

 Lehmann found that whilst his own blood in a normal state contained 4.14 parts 

 of chloride of sodium in 1000, this proportion was only increased to 4.15 after 



1 It is interesting to remark that Gorup-Besanez has found Silica to be a uniform com- 

 ponent of the feathers of Birds ; of the ashes of which it constitutes from 6.9 to as much 

 as 65 per cent. Moreover, the silica presents itself in much larger amount in the feathers 

 of adult birds than in those of the young, only traces of it being found in newly-grown 

 feathers ; and further, it is in the feathers of the wings, in which the greatest rigidity is 

 required, that the largest amount of silica is contained. 



2 This acid can always be obtained from gastric fluid by distillation. It has been found, 

 however, that lactic acid, which is also present in the stomach ($ 49), has the power of 

 decomposing chloride of sodium, and of setting free hydrochloric acid, by the aid of heat ; 

 but whether it ordinarily does this at the temperature of the stomach, is doubtful. Even 

 during evaporation in vacuo, lactic acid will decompose chloride of calcium, and will thus 

 disengage hydrochloric acid. 



