210 THE CIRCULATION OF THE BLOOD 



the heart must of necessity affect the general blood pressure and, therefore, 

 not directly any particular part. 



The general elasticity of the blood-vessels, and of the arteries in par- 

 ticular, which makes the general arterial pressure possible, is dependent 

 primarily on the presence of a large amount of elastic connective tissue in 

 the walls of the vessels. The elasticity of this tissue is a purely passive 

 property which can be utilized only by some positive source of energy, in 

 this instance the heart. 



The Variations in Peripheral Resistance. Certain arteries and 

 veins, especially the smallest ones, the arterioles, are supplied with muscular 

 tissue in their walls. The activity of these muscles in the vascular com- 

 plex makes the peripheral regulation of the flow of blood possible. They 

 supply a tissue which not only exhibits a passive elasticity comparable to that 

 of the yellow elastic connective tissue, but upon the proper stimulation they 

 actively contract or relax, thus securing to the peripheral resistance an active 

 adjustment to the ever- varying dynamic conditions of the vascular apparatus. 



The muscular tissue in the walls of the vessels increases relatively in 

 amount as the arteries become smaller, so that in the arterioles it is developed 

 out of all proportion to the other elements. In fact, in passing from the 

 arterioles to the capillary vessels, made up as we have seen of endothelial 

 cells with a supporting ground substance only, the last change on the side 

 of the arteries, which occurs as the vessels become smaller, is the disappear- 

 ance of muscular fibers. 



The office of the muscular coat is to adjust the size of the arterioles and, 

 therefore, the flow of the blood, to regulate the quantity of blood to be received 

 by each part or organ, and to adjust the quantity to the requirements of each, 

 according to various circumstances, but chiefly according to the degree of 

 activity which each organ at different times exhibits. The amount of work 

 done by each organ of the body constantly varies, and the variations often 

 quickly succeed each other, so that, as in the muscles for example, within 

 the same hour a part may be now very active and now quite inactive. In 

 all its active exercise of function, such an organ requires a larger supply of 

 blood than is sufficient for it during the times when it is comparatively 

 inactive. 



It is evident that the heart cannot regulate the blood supply to each part 

 of the body at different periods independently of the other parts. Neither 

 could this be regulated by any general and uniform contraction of the arteries. 

 But it may be regulated by the power which the arteries of each part have, 

 through their muscular tissue, of contracting or relaxing so as to diminish 

 or increase the supply of blood, according to the requirements of the par- 

 ticular part of the body to which the vessels are distributed. Thus, while 

 the ventricles of the heart determine the total quantity of blood to be sent 

 onward at each contraction, and the force of its propulsion, and while the 



