LOCAL VARIATIONS. 



305 



Fig. 101, 



in the capillaries of the general system, from red to blue. But its 



tinge also varies very considerably in different parts of the general 



circulation. The blood of the hepatic 



veins is darker than that of the femoral 



or brachial vein. In the renal veins 



it is very much brighter than in the 



vena cava ; and when the circulation 



through the kidneys is free, the blood 



returning from them is nearly as red 



as arterial blood. 



We must regard the circulation of 

 the blood, therefore, not as a simple 

 process, but as made up of many differ- 

 ent circulations, going on simultane- 

 ously in different organs. It has been 

 customary to illustrate it, in diagram, 

 by a double circle, or figure of 8, of 

 which the upper arc is used to repre- 

 sent the pulmonary, the lower the gen- 

 eral circulation. This, however, gives 

 but a very imperfect idea of the entire 

 circulation, as it really takes place. It 

 would be much more accurately re- 

 presented by such a diagram as that 

 in Fig. 101, in which its variations 

 in different parts of the body are 

 indicated in such a manner as to show, 

 in some degree, the complicated cha- 

 racter of its phenomena. The circula- 

 tion is modified in these different parts, 

 not only in its mechanism, but also in 

 its rapidity and quantity, and in the 

 nutritive functions performed by the 

 blood. In one part, it stimulates the 

 nervous centres and the organs of 

 special sense ; in others it supplies the 

 fluid secretions, or the ingredients of D ia?ram of t i, e 

 the solid tissues. One portion, in H rr - 2 L " n ^ 3 Head and nppe T 



* extremities. 4. Spleen. 5. Intestine. 6. 



paSSing tlirOUgh the digestive appara- Kiduey. 7. Lower extremities. S. Liver. 



tus, absorbs the materials requisite 



for the nourishment of the body ; another, in circulating through 

 20 



