186 THE CIRCULATION OF THE BLOOD 



the heart. With these endings poisoned, stimuli arising in the inhibitory 

 center of the medulla (tonic activity), or artifically applied to the vagus, 

 cannot reach the heart muscle, and inhibition is impossible. 



Muscarine, which is obtained from various species of poisonous fungi, 

 produces marked slowing of the heart-beats, and, in larger doses, stoppage 

 of the heart. It produces an effect similar to that of prolonged vagus stimu- 

 lation. The effect can be removed by the action of atropine, hence is 

 supposed to stimulate the nerve endings of the vagus. 



Digitalis slows the heart by stimulating the vagi at their origin in the 

 inhibitory center in the medulla. The heart muscle itself is also rendered 

 more excitable. 



Veratrine and aconitine have a somewhat similar effect. 



THE CIRCULATION THROUGH THE BLOOD-VESSELS. 



Blood Pressure. The subject of blood pressure has been already 

 incidentally mentioned more than once in the preceding pages ; the time has 

 now arrived for it to receive more detailed consideration. 



That the blood exercises pressure upon the walls of the vessels containing 

 it is due to the following facts: 



The heart at each contraction forcibly injects a considerable amount of 

 blood, 80 to 100 c.c., suddenly and quickly into the arteries. 



The arteries are highly distensible and stretch to accommodate the extra 

 amount of blood forced into them. The arteries are already full of blood 

 at the commencement of the ventricular systole, since there is not sufficient 

 time between the heart-beats for the blood to pass into the veins. 



There is a distinct resistance interposed to the passage of the blood from 

 the arteries into the veins by the enormous number of minute vessels, small 

 arteries (arterioles) and capillaries, into which the main artery has been 

 ultimately broken up. The sectional area of the capillaries is several hundred 

 times that of the aorta, and the friction generated by the passage of the blood 

 through these minute channels opposes a considerable hindrance or resistance 

 in its course. The resistance thus set up is called peripheral resistance. 

 The friction is greater in the arterioles, where the current is comparatively 

 rapid, than in the capillaries, where it is slow. 



The interaction of these factors heart-beat, elastic vessels, and periph- 

 eral resistance is sufficient to maintain a continuous flow of blood through 

 the entire circulatory system. It is the interrelation of these factors which 

 maintains an even and steady flow through the capillaries and past the tissues, 

 where it is desirable that the conditions of blood flow should be most con- 

 stant. In fact, we shall find that it is through the interaction of this same 

 group of factors, together with the possibility of variations through the regu- 

 lation of their nerve-motor mechanisms, that we have the great variations 



