100 CHEMICAL COMPONENTS OP THE HUMAN BODY. 



stances found in the excretions, one of whose constituents has been generated by 

 the metamorphosis of alimentary matters. And lastly, we shall notice those 

 substances which appear to be only accidentally present in the body. 



74. First, in order of importance whether we look at the large proportion 

 of the bulk of the fabric which is formed by it, to the influence which its pre- 

 sence exerts on the physical properties of the various tissues into which it enters, 

 or to the number and variety of purposes to which it is subservient in the chemico- 

 vital operations of the living body is unquestionably Water. The quantity 

 of this liquid which may be evaporated from the body by complete desiccation, 

 is, according to the recent experiments of Chevreul, 1 about two-thirds of its 

 entire weight ; and its predominance is by no means restricted to what are com- 

 monly accounted the " fluids" of the system, such as the Blood, Chyle, Lymph, 

 &c. ; since, as the following Table will show, 8 it is contained in nearly as large 

 an amount in several of the so-called " solid" tissues. 



Specific Gravity. 



1200 to 1600 



1300 



1200 



1100 



1100 



1080 to 1090 



1070.. 



1055 to 1060 



1050 to 1055 

 1040 to 1050 

 1030 to 1045 

 942 



When we examine into the uses of this large proportion of Water, we find, in 

 the first place, that it serves a purpose simply mechanical ; imparting to the 

 tissues that suppleness and extensibility which characterize them in their natural 

 state, but which are completely removed by drying them. Thus a piece of tendon, 

 when desiccated, shrinks into a firm and nearly inflexible rod, much resembling 

 a piece of dried glue ; yet, if macerated in water for a sufficient length of time, it 

 recovers, by the absorption of liquid, its original pliancy. In like manner, a 

 piece of the yellow fibrous tissue of the ligamentum nuchae of a Sheep or Ox dries 

 into a hard unyielding substance ; yet if allowed to imbibe its original proportion 

 of water it recovers its peculiar elasticity. The tissues in which we find least 

 water are those whose functions are most purely p Tiy sical; thus we see that 

 Bone, whose sole office is to afford an inflexible support, contains no more than 

 from 10 to 20 per cent, of fluid, the principal part even of this belonging to the 

 softer tissues immediately connected with its nutrition ; so in the Cuticle and its 

 appendages, whose purpose is merely protective, and which are partly desiccated 

 by exposure to the air, the proportion of solid matter is at least half. On the other 

 hand the proportion of water in Muscle averages 75 per cent., and in the sub- 

 stance of the Brain it is no less than from 80 to 84 per cent. ; the last-named 

 tissues being among those of which the vital endowments are the most remark- 

 able, and being those (as will be seen hereafter) in which the most rapid nutri- 

 tive changes take place during their state of vital activity. 



75. But further, the presence of Water is essential to the performance of all 

 those chemico-vital processes by which the integrity of the living body is main- 

 tained; and a deficiency in the aqueous portion of the fluids soon manifests itself 



1 " Anatomie General," par P. A. Beclard, 3ieme edit. 1851, p. 53. 



2 See Prof. Allen Thomson's " Outlines of Physiology," p. 130. 



