330 THE CIRCULATION. 



city ; and lastly the ventricular contraction takes place, driving the blood 

 into the large arteries. These movements continue to alternate with 

 each other, and form, by their recurrence, the successive cardiac pul- 

 sations. 



ft* 



The Arterial Circulation. 



The arteries are a series of branching tubes, which commence with 

 the aorta and ramify throughout the body, distributing the blood to the 

 various vascular organs. They consist of three principal coats, namely, 

 an inner coat, composed of thin elastic laminae lined with a single layer 

 of narrow, elongated and flattened epithelium cells ; a middle coat, com- 

 posed of elastic tissue and imstriped muscular fibres, running trans- 

 versely, or in a circular direction, round the calibre of the vessel; and 

 an external coat, consisting mainly of a more or less condensed layer 

 of connective tissue. The principal anatomical distinction between the 

 larger and the smaller arteries is in the structure of their middle coat. 

 In the smaller arteries this coat is composed exclusively of muscular 

 fibres, arranged in one or several layers. In arteries of medium size 

 the middle coat contains both muscular and elastic tissue; while in 

 those of the largest calibre it consists of elastic tissue alone. The 

 large arteries, accordingly, possess a remarkable degree of elasticity and 

 but little contractility; while the smaller are contractile, and less dis- 

 tinctly elastic. 



Movement of the Blood through the Arterial System. The movement 

 of the blood through the arteries is due to the muscular force of the 

 heart and the impulse derived from the ventricular systole. The arte- 

 rial system, which is an extensive ramification of tubular canals, may 

 be regarded as a great vascular cavity, subdivided from within outward 

 by the successive branching of its vessels, but communicating freely 

 with the heart and aorta at one extremity, and with the capillary plexus 

 at the other, and filled everywhere with the circulating fluid. At the 

 time of the heart's contraction, the muscular walls of the ventricle close 

 in upon its cavity ; and as the auriculo-ventricular valves at the same 

 time shut back and prevent regurgitation, the blood is forced out from 

 the ventricle through the aortic orifice. As the ventricle relaxes it is 

 again filled with blood from the auricle, and delivers it, as before, by 

 a new contraction, into the arteries. It is by these impulses, recurring 

 at short intervals, that the entire blood moves in a direction from the 

 heart outward through the arterial system. 



Distension of the Arteries by the HearVs Action; Arterial Pulse. At 

 each ventricular systole a charge of blood is driven into the arteries, 

 distending their walls by the pressure of the additional quantity of fluid 

 introduced into their cavities. When the ventricle afterward relaxes, 

 this active distending force is suspended ; and the elastic arterial walls, 

 reacting upon their contents, would drive the blood back into the heart 

 were it not for the closure of the semilunar valves, which shut together 



