CIRCULATION OF THE BLOOD. 525 



The direction of the muscular fibres of the arteries is circular, while 

 longitudinal fibres are absent. As a consequence, the contraction of the 

 muscular coat of an arterj- tends to obliterate its calibre. On the other 

 hand, the elastic element tends to keep the artery open. When, there- 

 fore, an artery is reduced in calibre by contraction of its muscular fibres, 

 when the muscular coat becomes relaxed, the elastic coat dilates it. 

 There is no active dilating mechanism in the walls of the blood-vessels. 

 The combined action of these two forces, the expanding force of the 

 elastic coat and the contracting force of the muscular coat, would serve 

 to cause the arteries to assume the form of hollow ribbons with flattened 

 sides, or flattened cylinders. This shape is found in the arteries of an 

 animal when examined after death. In the act of dying the arteries 

 empty themselves by the contraction of the muscular coat, forcing their 

 entire contents over into the venous system ; they, therefore, become 

 completely emptied, and are then flattened cylinders; this condition 

 holds until one of the larger arteries be opened ; air then enters the 

 arteries, the muscular force having been lost through death; the arteries 

 then dilate through the action of the elastic tissue which is longer pre- 

 served, and they now become hollow cylinders filled with air. It is thus 

 seen that the larger arteries are highly elastic tubes, and the influence of 

 the elasticity of the walls of a tube on a moving column of fluid has been 

 already alluded to. In other words, the elastic tissue in the walls of the 

 large arteries tends to overcome the intermittent action of the heart and 

 to render the flow of blood in the arteries continuous. In the smaller 

 arteries the elastic tissue is reduced in amount and often becomes en- 

 tirely absent, but, on the other hand, the proportion of muscular tissue is 

 increased. Muscular tissue is itself a highly elastic tissue, consequently 

 the smaller arteries are not only elastic but are also supplied with con- 

 tractile walls, and as a consequence their calibre maybe reduced, thus 

 permitting variations in the supply of blood to different localities. 



The conditions for permitting a satisfactory interchange between 

 the blood and different organs are, therefore, fulfilled. For we have not 

 only a constant flow of blood through all parts of the body, but this flow 

 is susceptible of general and local alterations ; general alterations, be- 

 cause the heart itself is capable, as already indicated, of being modified 

 in its activity ; and, second, because we see that the smaller arteries are 

 supplied with tissue which, by regulating the calibre of the blood-vessels, 

 is capable of regulating the amount of blood supplied to different organs. 

 The mechanism by which this supply is governed will be alluded to 

 directly. 



Blood Pressure. — In the arteries in their normal state the elastic 

 coat is in a condition of distention beyond its point of equilibrium. In 

 other words, the arteries are vessels overfilled with fluid. The contents 



