THE ARTERIES AND THE ARTERIAL CIRCULATION. 281 



protrudes, strikes against the walls of the chest, and twists from 

 left to right, the auriculo-ventricular valves shut back, the first 

 sound is produced, and the blood is driven into the aorta and 

 pulmonary artery. These phenomena occupy about one-half the 

 time of an entire pulsation. Then the ventricle is immediately 

 relaxed, and a short period of repose ensues. During this period 

 the blood flows in a steady stream from the large veins into the 

 auricle, and through the auriculo-ventricular orifice into the ven- 

 tricle ; filling the ventricle, by a kind of passive dilatation, about 

 two-thirds or three-quarters full. Then the auricle contracts with a 

 quick sharp motion, forces the last drop of blood into the ventricle, 

 distending it to its full capacity, and then the ventricular contraction 

 follows, as above described, driving the blood into the large arteries. 

 These movements of contraction and relaxation continue to alter- 

 nate with each other, and form, by their recurrence, the successive 

 cardiac pulsations. 



THE ARTERIES AND 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 all the vascular organs. They are composed of three 

 coats, viz : an internal homogeneous tunic, continuous with the 

 endocardium; a middle coat, composed of elastic and muscular 

 fibres ; and an external or " cellular" coat, composed of condensed 

 layers of areolar tissue. The essential anatomical difference be- 

 tween the larger and the smaller arteries consists in the structure 

 of their middle coat. In the smaller arteries this coat is composed 

 exclusively of smooth muscular fibres, arranged in a circular man- 

 ner around the vessel, like the circular fibres of the muscular coat 

 of the intestine. In arteries of medium size the middle coat con- 

 tains both muscular and elastic fibres ; while in those of the largest 

 calibre it consists of elastic tissue alone. The large arteries, ac- 

 cordingly, possess a remarkable degree of elasticity and little or no 

 contractility ; while the smaller are contractile, and but little or not 

 at all elastic. 



It is found, by measuring the diameters of the successive arte- 

 rial ramifications, that the combined area of all the branches given 

 off from a trunk is somewhat greater than that of the original 

 vessel ; and therefore that the combined area of all the small arte- 

 ries must be considerably larger than that of the aorta, from which 



