140 CIRCULATING FLUIDS: 



and respiration^ inasmuch as, without them, the blood would 

 not be brought in contact with the oxygen, which is necessary 

 for the existence of life ; and the more completely these func- 

 tions are discharged, the more perfectly will the due changes 

 in the blood be effected ; if, on the contrary, the blood is de- 

 tained in any part of the body, or cannot enter the sphere of at- 

 mospheric action in the lungs, the metamorphosis can be only 

 imperfectly effected. 



We know, from the investigations of Schwann and Reichert, 

 that all the tissues of the animal body are composed of cells, and 

 that nutrition and growth of the organs and tissues is conducted 

 by the production of new cells, appropriate for each individual 

 organ, developing themselves at every point where the substance 

 from which they are formed, viz. the blood, is conveyed ; that 

 these cells, by their organic formation, effect a change in the 

 nutritious plasma, by appropriating from it matters homologous 

 to themselves, and that the cells are finally consumed or dis- 

 solved, as is obvious from the general phenomena of the circu- 

 lation. The nutrition and consumption of the tissues of the 

 animal body in the general process of life is, consequently, the 

 product of the nutrition and consumption of the cells which 

 constitute those tissues. Since the capillaries are distributed 

 over every particle of each individual tissue, and since their walls 

 are composed of cells, which can communicate and impart the 

 plasma to the adjacent cells, the plasma can be universally dis- 

 tributed, and the reciprocal action between it and the cells of 

 the various organs ensured. 



In what manner the cells act upon the nutrient fluid we are not 

 able to understand, but there can be little doubt that they, or 

 (which amounts to the same thing) the organs and tissues which 

 they constitute, produce adialytic, catalytic, or, as Schwann terms 

 it, a metabolic change on the plasma of the blood. The products 

 of these influences must necessarily consist of certain chemical 

 compounds, formed in very different ways, and varying in their 

 nature in accordance with the activity of the nervous power. 

 The high atomic numbers of those animal substances which are 

 of the most importance in nutrition, as the protein-compounds 

 and fats, render the existence of numerous decompositions ex- 

 tremely probable. In vegetable chemistry we find whole classes 

 of substances transmutable, one into the other, in which the 



