THE CIRCULATION. 181 



action to the intrinsic ganglia of the heart. These gan- 

 glia with their nerves leading to the muscle cells inaug- 

 urate the heart beat. To control this heart beat two kinds of 

 nerves reach it from the exterior: first, the vagi or pneumo- 

 gastric nerves, which because they slow, and if sufficiently 

 stimulated, stop the heart beat, are called cardio-inhibitory 

 nerves. These are normally in constant excitation; and so 

 the heart beat is always a little slower than it would of its 

 own account be. Second, the sympathetic or accelerator 

 nerves reaching it by way of the sympathetic ganglia, the 

 physiological action of which is to increase the rate of the 

 beat. 



All these nerves, the intrinsic, cardio-inhibitory and the 

 sympathetic, are concerned in the movement of the heart. 

 They are not sensory. But in addition to these three motor 

 nerves an afferent nerve reaches the heart carrying sensa- 

 tions to the medulla. This afferent nerve, because when it 

 is stimulated by conditions in the heart, it causes a dilatation 

 of the blood vessels in all parts of the body, is called the 

 depressor nerve. The dilatation of the arteries everywhere 

 is, of course, not due to the depressor nerve itself; it 

 simply carries the afferent impulse to the medulla, and from 

 centers in the .medulla the impulses to dilate the arteries 

 arise. 



Finally, it need not here be noted that these nerves are 

 in pairs, a pneumogastric on each side and a sympathetic 

 right and left. 



THE DYNAMICS OF THE BLOOD STREAM IN THE VESSELS. 



1. The Blood Pressure. The anatomy and the general 

 distribution of arteries, veins and capillaries have already 

 been given; but there still remains the discussion of the 

 phenomena of the blood in motion. By the beat of the heart 

 the blood is thrown into the arteries, where it accumulates 

 until it reaches a certain pressure sufficient to force it out 

 through the small capillaries at the peripheral ends. The 

 most apparent observation probably in examining an artery 



