166 



divisions, forming one-half of the circle; the pulmonary veins, the left 

 cavities of the heart, the aorta with all its branches representing the 

 other half. The capillary vessels of the lungs form one of the points of 

 intersection, and the capillaries of all the other organs, represent the other 

 point of intersection, by uniting together the arteries and veins of the 

 whole body, in the same manner as those of the lungs establish a com- 

 munication between the veins and arteries of these organs. 



This division of the system of circulation into two parts, in one of which 

 there circulates a dark or venous blood, while the other contains red or 

 arterial blood, is at once more simple and more accurate. As was alrea- 

 dy stated, in the history of the circulation, that its organs are, in an espe- 

 cial manner destined to the mechanical act of conveying the fluids: the 

 changes, the alterations which the blood undergoes in passing through 

 the organs, are affected, only at the moment when in penetrating into 

 their tissue, it passes into the capillary vessels which are distributed 

 into them. The columns of blood are then sufficiently minute, to be 

 operated upon by the vital action; till then, the columns of blood are too 

 large, and resist, by their bulk, if one may so speak, any decomposition. 

 It is, therefore, in the capillary vessels, that the blood receives its essen- 

 tial principles; and to understand how the nutritious lymph which is 

 deposited by the thoracic duct in the left subclavian vein, experiences, 

 in its course along the sanguiferous system, the changes which are to 

 assimilate it to our own substance, it is necessary to follow it, along the 

 venous blood with which it unites, into the heart, through the right half 

 of which it passes in its way to the lungs, there to combine with the at- 

 mospherical air, from which we are perpetually deriving another aliment 

 indispensable to life; then to examine, how, when modified and conveyed 

 with the red blood, from the lungs to the whole body, it serves to the 

 secretions, and supplies nourishment to the whole body. 



In considering, in this manner, the circulation of the blood, with a re- 

 ference to the changes which it undergoes in the organs through which 

 it passes, in describing that circle, we shall find, that this fluid, already 

 combined with the lymph and chyle, parts, in the lungs, with some of its 

 principles, at the same time that it becomes impregnated with the vital 

 portion of the atmosphere, which suddenly changes its colour and other 

 qualities. The blood will then be seen to flow into a!l the parts which it 

 stimulates, to keep up their energy, to awaken their action, and furnish 

 them the materials of the fluids which they secrete, or the molecules by 

 which they grow or are repaired; so that in supplying thus the different 

 organs, the blood loses all the qualities which it had acquired by the union 

 of the chyle and of the vital air, parts with the principles to which it 

 owed its colour, and again becomes dark, to be repaired anew by com- 

 bining with the lymph, and by the absorption of the vital part of the at- 

 mospherical air*: this constitutes the principal phenomenon of the func- 

 tion, which will be considered in the fourth chapter. 



* See the Notes on Respiration, in the APPENDIX, for a different opinion. 



