WASTE OF TISSUES. 123 



parts by the cohesion of which the epidermis is constituted. 

 And the same thing may be said of every vital part in contrast 

 with the epidermis ; the minute parts which compose any one 

 living solid are all extravascular ; they are as much a deposit 

 from the blood as the cells of the epidermis. Thus there is 

 nothing in any solid or in any fluid which was not once in the 

 blood that is to say, the elements of which at least were not 

 once in the blood ; and there is nothing in the blood which 

 was not once in the aliment, except such slight material as may 

 be absorbed by the skin or lungs. 



To maintain the blood, then, in its normal condition, is the 

 great object of nutrition, and such normal condition is that 

 state in which it is fit to supply every necessary constituent to 

 the solids and fluids as often as a deficiency has arisen. The 

 expenditure of the blood is most manifestly for the augmenta- 

 tion and repair of the solids and fluids. That expenditure is 

 greatly increased under violent or long-continued exertion of 

 the muscular frame. Every contraction of a muscular fibril is 

 accompanied with a partial disorganisation of its living sub- 

 stance that is, with the conversion of part of its living texture 

 into non-vital chemical compounds, which must be thrown 

 forth as effete, and therefore, if retained, are useless or hurtful 

 to the animal economy. This continual disintegration of the 

 muscular solid, or conversion of it into inert chemical products, 

 would, it is to be supposed, furnish the same variety of sub- 

 stances which are produced when a piece of muscle is subjected 

 to destructive distillation, while the influence of the atmos- 

 phere is excluded ; but what really occurs is similar to the 

 effect produced when such an animal solid is subjected to 

 destructive distillation after being mingled with a substance 

 like oxide of copper, capable of supplying oxygen in abun- 

 dance, because the contraction of a muscular fibril always takes 

 place in the presence of the abundant oxygen contained in the 



