CHAPTER XVII. 



THE NERVES OF THE HEART, AND SOME PHYSIOLOGI- 

 CAL PECULIARITIES OF CARDIAC MUSCLE. 



The Co-ordination of Heart and Arteries. We have 

 hitherto considered the working of the vascular system as if 

 it were a mere mechanical hydraulic apparatus; and such in 

 a certain sense it is, and by so regarding it many of the phe- 

 nomena of the blood-flow can be explained. But life is a 

 constant adjustment to constantly varying conditions, and 

 the higher the organism the more numerous the conditions 

 which influence it and the greater its power of adapting itself 

 to them ; and this adaptability, this continuous self -adjust- 

 ment, is nowhere better exhibited than in the heart and 

 blood-vessels. 



The object to be attained is the maintenance of an orderly 

 current in the capillaries in accordance with the needs of the 

 whole Body and of each of its organs at the time. This 

 clearly calls for some means -of interaction between heart anu 

 blood-vessels: should the heart beat and the arteries relax or 

 contract, each without reference to the other, no steady capil- 

 lary flow could result. To secure such a flow the work done 

 by the heart and the resistance offered in the vessels must 

 at all times be correlated; so that the heart shall not 

 by too powerful action over-distend or perhaps burst the 

 small arteries, nor the latter contract too much and so, by 

 increasing the peripheral resistance, raise aortic pressure to a 

 great height and increase unduly the work to be done by the 

 left ventricle in forcing open the semilimar valves. 



Further, the total amount of blood in the Body is not suffi- 

 cient to keep all its organs simultaneously supplied with the 

 amount needful for the full exercise of their activity; in fact 

 the blood-vessels of the spleen, liver, and alimentary canal, if 

 all fully distended, can themselves contain almost the whole 

 blood of the Body, so that by paralyzing their coats in an animal 

 it can be caused to faint, or even be killed, by what has been 



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