THE CIRCULATION OP THE BLOOD. 265 



the blood-corpuscles no longer pursue their usual course in the 

 mid-stream, but seem to be generally distributed and to hug the 

 walls, one result of which is a slowing of the stream, wholly 

 independent of events taking place in other vessels. It is thus 

 seen that in this condition (stasis) the capillaries have an in- 

 dependent influence essentially vital. We say independent, for 

 it is still an open question whether nerves are distributed to 

 capillaries or not. That inflammation, in which also the walls 

 undergo such serious changes that white and even red blood- 

 cells may pass through them (diapedesis), is not uninfluenced 

 by the nervous system, possibly induced through it in certain 

 cases, if not all, seems more than probable. 



But when we consider the lymphatic system new light will, 

 it is hoped, be thrown upon the subject of the nature and the 

 influences which modify the capillaries. One thing will be 

 clear from what has been said, that even normally the capil- 

 laries must exert an influence of the nature of a resistance, 

 owing to their peculiar vital properties ; and, as we have already 

 intimated such considerations should not be excluded from any 

 conclusions we may draw in regard to tubes that are made up 

 of living cells, whether arteries, veins, or capillaries, though 

 manifestly the applicability to capillaries, with their less modi- 

 fied or more primitive structure, is stronger. 



It has now become clear that the circulation may be modi- 

 fled either centrally or peripherally; that a change is never 

 purely local, but is correlated with other changes ; that the 

 whole is, in the higher animals, directly under the dominion of 

 the central nervous system ; and that it is througb this part 

 chiefly that harmony in the vascular as in other systems and 

 with other systems is established. To have adequately grasped 

 this conception is worth more than a knowledge of countless 

 details. 



SPECIAL CONSIDERATIONS. 



Pathological. — Changes may take place either in the sub- 

 stance of the cardiac muscle, in the valves, or in the blood-ves- 

 sels, of a nature unfavorable to the welfare of the body. Some 

 of these have been incidentally referred to already. 



Hypertrophy, or an increase in the tissue of the heart, is 

 generally dependent on increased resistance, either within or 

 without the heart, in the region of the arterioles or capillaries. 

 Imperfections of the aortic valves may permit of regurgitation 



