700 SPECIAL PHYSIOLOGY. 



Nutrition of the Blood. 



The most obvious evidence of organization in the blood is the exis- 

 tence of its white and red corpuscles. The very special characters of 

 the latter, which are found only in the blood of the Vertebrata, and 

 which, as we shall see, undergo evolution and decay, afford proof of a 

 further assimilative action than that which takes place in the formation 

 of the lymph and chyle. The blood, however, owes the maintenance 

 of its extremely composite nature to other processes than to those of 

 a self-assimilative character: viz., to its losses in the nutrition of the 

 solid tissues of the body, and in the formation of the various secretions ; 

 to the changes which it incessantly undergoes, by the entrance into it 

 of new matter, by venous absorption on the one hand, and to the pas- 

 sage from it of effete matter, by excretion on the other; and lastly, to 

 the profound alterations in its character, effected by the action of oxygen 

 in the respiratory process. For these reasons, the subject of the for- 

 mation of the blood, or Sanguification, will be more conveniently con- 

 sidered hereafter. 



Nutrition of the Organs and Tissues. 



The nutrition of the various solid parts of the body, nutrition proper, 

 or secondary assimilation, consists of the following steps or stages. 



First, a nutritive fluid or plasma exudes from the blood, through 

 the coats of the capillaries, into all the interstices of the tissues. This 

 process may be partly due to porous diffusion under the influence of 

 the pressure of. the blood in the bloodvessels, but partly also to a dialytic 

 movement, regulated by the chemical relations between the liquor san- 

 guinis and the walls of the capillary vessels. As these vessels are 

 everywhere developed in the same manner, from coalesced nucleated 

 cells, the structure, chemical composition, and properties of their walls 

 are probably the same in every tissue and organ. Hence we may 

 infer that the nature of the exuded fluid plasma, is the same in every 

 part of the body, and, accordingly, that a uniform material derived 

 from the blood, is provided for the nutrition of every part of the system. 

 This nutritive plasma is sometimes supposed to be identical with the 

 liquor sanguinis; but this is doubtful. Fluids effused into various parts 

 of the body, when present in sufficient quantity to be examined, are 

 found to undergo coagulation after they are removed from the influence 

 of the living tissues ; they therefore contain a certain quantity of fibrin 

 or fibrinogen, and so far resemble the liquor sanguinis; but they appear 

 generally to possess less albumen and saline matter. Moreover, the 

 liquor sanguinis contains all the ingredients of the blood, whether nu- 

 tritive or effete, excepting the white and red corpuscles; whilst it is 

 more probable that the exuded plasma, destined for the nutrition of 

 the tissues, consists of a purer nutrient material. Whatever its precise 

 nature, the exuded plasma passes into the finest interstices of the vas- 

 cular tissues, between the capillary networks, and bathes all the 

 elementary parts of those tissues. Moreover, it penetrates, beyond 



