30 INTRODUCTION. 



large as well as small, is composed chiefly of nitrogen, with 

 hydrogen and carburetted hydrogen in about equal propor- 

 tion, five to eleven parts per hundred, and but a trace of sul- 

 phuretted hydrogen. With the exception then of oxygen 

 and carbonic acid, the latter being an excretion, the gases do 

 not hold an important place among the proximate principles. 

 At all events, their function, whether it be important or not, 

 is but little understood. 



Water, HO. 



Water is by far the most important of the inorganic prin- 

 ciples. 1 It is present at all periods of life, existing even in 

 the ovum. It exists in all parts of the body ; in the fluids, 

 some of which, as the lachrymal fluid and perspiration, con- 

 tain little else, and in the hardest structures, as the bones, or 

 the enamel of the teeth. 



. In the solids and semi-solids it does not exist as water, 

 but enters into their structure, assuming the consistence 

 by which they are characterized. For example, we have 

 water in the bones, teeth, and even in the enamel, not con- 

 tained in the interstices of their structure, as in a sponge, 

 but incorporated into the substance of the tissue. In these 

 situations it is essentially water of composition. During the 

 process of nutrition, water is deposited in the tissues with the 

 other nutritive principles, as we have it incorporated in the 

 substance of certain inorganic compounds in the process of 

 crystallization, when it is known in chemistry as water of 

 crystallization. In the interior of the body, water is thus 

 incorporated in the substance of organic matters, which are 



1 In comparing principles which are essential to nutrition and to life, it is im- 

 possible to say that one is absolutely more important than another ; still, writers 

 are in the habit of making a distinction in the importance of necessary constit- 

 uents of the body, chiefly with reference to their quantity and the extent of their 

 distribution. When we come to organic principles, we shall see that these are 

 manifestly the most important constituents of the living body, as giving to the 

 tissues their vital properties. 



