56 MANUAL OF HISTOLOGY. 



alkalies, especially soda, combined with protein compounds, and also iron, 

 in many animal substances, as, for instance, in hsemoglobulin and me- 

 lanin. 



In regard to the gases just mentioned, they are found either in the cavi- 

 ties of the body, or diffused, or chemically combined in its various fluids. 



Oxygen, 0. 



Oxygen occurs in the organic matters of the animal body in combina- 

 tion. It appears, however, also as an element in all the air cavities of the 

 system. Finally, it is met with in all the fluids of the economy. In the 

 blood oxygen is dissolved in very minute quantity, while the greater por- 

 tion appears combined (though loosely) with the other constituents of the 

 fluid. We need hardly remark that this element, from its strong tendency 

 to combine with other substances, plays a most important part in the 

 chemical and physiological life of the organism. 



Nitrogen Gas, ET 2 . 



Nitrogen, as is well known, occurs in combination in many organic 

 matters in the body : it is also met with, however, in the air cavities of 

 the latter, and in very small quantities dissolved in its various fluids. 



Carbonic Acid, or Carbonic Dioxide, C0 2 . 



Carbonic acid appears partly in combination (especially with inorganic 

 bases), partly free, either as a gas, or dissolved in the fluids of the body. 

 As a gas, carbonic acid is present in considerable quantity in the gases 

 expired from the lungs, and in various cavities containing air. Dissolved, 

 it is a constituent (though in variable amount) of all animal fluids. It 

 appears in abundance in the blood, moreover, partly free and partly com- 

 bined. Carbonic acid, which is introduced into the economy in but small 

 amount from without, is the most important end-product of many chemical 

 mutation series in the body. It leaves the latter in large quantities 

 through the lungs, and to a small extent with the exhalations of the skin. 



40. 

 Water, H 3 . 



No inorganic compound is of such great importance for the existence 

 of the organism, or occurs throughout all its parts in such abundance, as 

 water : without it life is impossible. Setting aside that which occurs in 

 hydrates and in crystals, water is necessary to the organism, in the first 

 place, as a solvent for many of its constituents. By virtue of this property 

 it renders an interchange of material possible. Dissolved in water, the 

 alimentary matters are absorbed into blood and tissues, and by it effete 

 substances are carried out of the body. In the preceding section we have 

 already alluded to its power of absorbing gases. 



The proportion of water to the whole weight of the body is in general 

 very considerable ; in the higher animals at a period of maturity it is, on 

 an average, about 70 per cent., while in embryos it is still larger, ranging 

 from 87 to 90 per cent. In the infant and in younger animals its amount 

 gradually sinks, while that of solid organic and mineral matters undergoes 

 constant increase (Schlossberger, Bezold). That the proportion of water 

 in different parts of the body varies to an enormous extent is quite evident, 



