ELEMENTS OF COMPOSITION. 57 



and will be alluded to later on more in detail. For the present it need 

 only be remarked that, as water, on the one hand, renders possible all 

 the chemical occurrences of the body by virtue of its solvent power, so, 

 on the other hand, does it communicate to each tissue its individual 

 stamp, from a physical or physiological point of view, as an imbibed mat- 

 ter. Its amount in the soft, semi-solid portions of our body appears dis- 

 proportionally large, but even in the harder structures, such as bone, it 

 is not inconsiderable. 



Besides that which is generated within the body by oxydation from 

 the H of organic substances, water is introduced into the body with food, 

 both solid and liquid. 



Hydrochloric Acid, C1H . 



This acid is only found free in the gastric juice. 



Silicic Acid or Silicon Dioxide, SiO, . 



Very small quantities of silicic acid, either free or combined in salts, 

 have been met with in human blood (Millori), saliva, urine, bile, and 

 excrement, as well as in biliary and urinary calculi, bones, and teeth. 

 But of all the tissues of the human body, the hairs, according to Gorup- 

 Besanez, contain most of it. 



Silicic acid is taken into the body with the food and drinking water, 

 and passes out of the same, for the most part, immediately through the 

 intestinal canal, while a portion of it is absorbed into the blood, and 

 appears later in the secretions of the various glands. 



The physiological or anatomical significance of silica in the human 

 system is not known. 



11. 



Calcium Compounds. 



Lime, CaO, which next to soda is the most important inorganic base 

 of the body, presents itself in many different combinations. 



PHOSPHATE OF CALCIUM. 



Phosphoric acid occurs, as is well known, under various modifications, 

 of which, however, only the ordinary or tribasic acid appears in the 

 system. The following are its calcium salts : (a), Acid phosphate of 

 calcium, as it is called, CaH 4 P 2 8 ; (b), Neutral phosphate, CaHP0 4 ; and 

 (c), Basic phosphate, Ca ;i P 2 8 . 



Basic, Ca 3 P 2 8 , and neutral CaHP0 4 , phosphates of calcium. 



The first of these is almost insoluble in water, but to a certain extent 

 soluble in that containing carbonic or organic acids, as also in solutions 

 of ammonium salts, chloride of sodium, and of animal gelatin. It is, as we 

 have seen, the particular salt of calcium which occurs in the bones and 

 teeth, and probably exists besides widely distributed throughout the 

 animal body, while the acid salt is present in human urine. 



Phosphate of calcium, which has its origin in general from the alimentary 

 matters, appears in very variable amount in all the solid arid fluid portions 

 of the system. Wherever it is present in large quantities it is the most 

 important hardening agent of the latter. Its deposits are almost always 

 amorphous. 



Phosphate of calcium has been shown to exist in the blood, urine, gastric 



