CONDITIONS NECESSARY FOR NUTRITION. 389 



speaking of the power of assimilation which the blood 

 exercises, a power exactly comparable with this of main- 

 tenance by nutrition in the tissues. And evidence of the 

 adaptation between the blood and the tissues, and of the 

 exceeding fineness of the adjustment by which it is main- 

 tained, is afforded by the phenomena of diseases, in which, 

 after the introduction of certain animal poisons, even in 

 very minute quantities, the whole mass of the blood is 

 altered in composition, and the solid tissues are perverted 

 in their nutrition. It is necessary to refer only to such 

 diseases as syphilis, small-pox, and other eruptive fevers, 

 in illustration. And when the absolute dependence of all the 

 tissues on the blood for their very existence is remembered, 

 on the one hand, and, on the other, the rapidity with which 

 substances introduced into the blood are diffused into all, 

 even non- vascular textures (p. 376), it need be no source of 

 wonder that any, even the slightest alteration, from the 

 normal constitution of the blood, should be immediately 

 reflected, so to speak, as a change in the nutrition of the 

 solid tissues and organs which it is destined to nourish. 



2. The necessity of an adequate supply of appropriate blood 

 in or near the part to be nourished, in order that its nutrition 

 may be perfect, is shown in the frequent examples of 

 atrophy of parts to which too little blood is sent, of morti- 

 fication or arrested nutrition when the supply of blood is 

 entirely cut off, and of defective nutrition when the blood 

 is stagnant in a part. That the nutrition of a part may 

 be perfect, it is also necessary that the blood should be 

 brought sufficiently near to it for the elements of the tissue 

 to imbibe, through the walls of the blood-vessels, the 

 nutritive materials which they require. The blood-vessels 

 themselves take no share in the process of nutrition, except 

 as carriers of the nutritive matter. Therefore, provided 

 they come so near that this nutritive matter may pass by 

 imbibition into the part to be nourished, it is compara- 

 tively immaterial whether they ramify within the substance 



