78 PHYSIOLOGY. 



being the occasion of a disordered nutrition of certain parts of the 

 skin ; and the selecting power of particular spots being evinced by 

 the exact correspondence of the parts affected on the two sides. 



In the process of nutrition is exemplified the fundamental principle 

 of organic assimilation. Each elementary particle of an organ attracts 

 similar particles from the blood ; and by the changes it produces in 

 them, causes them to participate in the vital principle of the organ 

 itself. Nutrition does not consist merely in the component particles 

 of the organs attracting the fibrin, albumen, and other materials of the 

 blood which flow through them, adding to themselves the matters 

 similar to their own proximate principles, and changing the compo- 

 sition of those which are dissimilar ; but the assimilating particles 

 must infuse into those newly added to them their own vital proper- 

 ties. Mere increased size is not nutrition ; parts may be increased 

 in size by the deposition of fibrin, as in inflammation, but this fibrine 

 is unassimilated, and not endowed with the vital properties of the 

 tissue in which it is deposited, and in this consists the difference be- 

 tween increased nutrition and increased size. It was said that the 

 source of all nutrition was the blood : a short account of this fluid 

 may be necessary here. The chyle and the lymph, which are also 

 generally considered as nutritive fluids, have been already described 

 under the head of absorption. 



The blood, whilst circulating in the living vessels, consists of two 

 parts, — a thin, transparent, nearly colourless liquid, termed liquor 

 sanguinis ; and a number of small bodies called the red corpuscles, 

 from which the colour of the blood of vertebrated animals derives 

 its peculiar hue ; in addition to which are found some white, or colour- 

 less corpuscles. 



When the blood has been drawn from the body and allowed to 

 stand, a spontaneous coagulation takes place, dividing it into crassa- 

 mentum and serum. The crassamentum, or clot, is formed by the 

 union of the fibrine and red corpuscles, by the entanglement of the 

 latter in the meshes of the former. Whilst the serum is merely the 

 liquor sanguinis deprived of its fibrine. From the fact that the 

 serum coagulates by the addition of heat we know that it contains 

 albumen ; by exposure to a high temperature, the animal matter is 

 decomposed, and a considerable quantity of earthy and alkaline salts 

 remain. 



The distribution of these constituents in living and dead blood may 

 be seen in the following table. 



/ Fibrine, i 

 .... T»i J ) Albumen, > In solution, formingf Liquor Sanguinis. 

 Living Blood, j g^j^^^ ' S S 4 ^ 



( Corpuscles. — Suspended in Liquor Sanguinis. 

 ( Fibnne, ) Crassamentum, or clot 

 Dead Blood. \ ^^Zi''' 5 



( Salts < ^" solution, forming serum. 



