490 NUTRITION. 



other gases is by no means so essential. Nitrogen seems to be formed by the system in 

 small quantity and is taken up by the blood and exhaled by the lungs, except during inani- 

 tion, when the blood absorbs a little from the inspired air. It exists in greatest quantity 

 m the intestinal canal. Oarburetted and sulphuretted hydrogen, with pure hydrogen, 

 are found in minute quantities in the expired air and are also found in a gaseous state in 

 the alimentary canal. From the offensive nature of the contents of the large intestine, 

 we should suspect the presence of sulphuretted hydrogen in considerable quantity; but 

 actual analysis has shown that the gas contained in the stomach and intestines, large as 

 well as small, is composed chiefly of nitrogen, with hydrogen and carburetted hydrogen 

 in about equal proportions (five to eleven parts per hundred), and but a trace of sulphu- 

 retted 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. This principle exists in all parts of the body ; in the fluids, some of which, 

 as the lachrymal fluid and perspiration, contain little else, and in the hardest structures, 

 as the bones and the enamel of the teeth. In the solids and semisolids it does not exist as 

 water, but it enters into their structure, assuming the consistence by which the tissues are 

 characterized. For example, we have water in the bones, teeth, and even in the enamel, 

 not contained 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 mat- 

 ters, which are of indefinite chemical composition and non-crystallizable, and the water 

 enters into their composition, within certain limits, in indefinite proportions, assuming 

 the consistence of the organic substance. As physiologists, studying the organism not 

 from a purely chemical point of view, we must consider water as an integral constituent 

 of the tissues and not as merely absorbed by them. 



All the organized structures contain a certain proportion of water, and this is neces- 

 sary to the performance of all or any of their functions. If a normal muscle be consid- 

 ered as a contracting organ, and a nerve, as a conducting organ, or albumen, as a nutri- 

 tious element, we must consider water as one of their constituents. It is necessary to 

 the proper form, consistence, and function of these and of all organized structures. In 

 analyses of organic matters, when water is lost or driven off in our manipulations, the 

 principle is not brought near a state of chemical purity, but it is essentially and radically 

 changed. 



The quantity of water which each organic substance contains is important ; and it is 

 provided that this quantity, though indefinite, shall not exceed or fall below certain lim- 

 its. The truth of this proposition is made evident from the following facts : In the first 

 place, all organs and tissues must contain a tolerably definite quantity of water to give 

 them proper consistence. The evils of too great a proportion of water in the system, 

 and consequently a diminution of solid elements, are well known to the practical physi- 

 cian. General muscular debility, loss of appetite, dropsies, and various other indications 

 of imperfect nutrition, are among the results of such a condition ; while a deficiency of 

 Avater is immediately made known by the sensation of thirst, which leads to its introduc- 

 tion from without. 



The fact that water never exists in any of the fluids, semisolids, or solids, without 

 being combined with inorganic salts, and especially chloride of sodium, is one reason why 

 its proportion in various situations is to a certain extent constant. The presence of these 

 salts influences, in the semisolids at least, the quantity of water entering into their com- 

 position, and consequently it regulates their consistence. A very simple experiment shows 



