GENERAL FEATURES OF THE CIRCULATION 981 



becomes arterial and is returned by the pulmonary veins to the left 

 auricle and so to the left ventricle. The rhythmic contractions of the 

 left ventricle then force the blood into the aorta, whence by the 

 branching arteries it is carried to all parts of the body. The whole 

 vascular system is distensible and elastic, so that its capacity will 

 increase with the pressure of the blood contained in it. Since the 

 driving force is furnished by the heart the pressure which causes the 

 flow of blood through the system must decline as we pass from the 

 arterial to the venous side. The chief function of the large arteries is to 

 serve as elastic conduits, whereas the small arteries or arterioles leading 

 from the arteries to the capillaries have in addition the function of 

 regulating the amount of blood flowing through the capillary area 

 of the organs which they supply. The veins have the function of 

 conducting blood at a low pressure from capillaries to heart and of 

 storing up any excess of blood which is not immediately taken up by 



FIG. 368. Transverse section of part of the wall of the posterior 



tibial artery ( x 75). 



a, endothelial and sub-endothelial layers of intima ; 6, lamina of elastic tissue ; 

 c, media consisting of muscle fibres ; d, adventitia. (ScHAFER.) 



the heart. Corresponding to this difference in function we find 

 variations in the structure of the blood-vessels according to their 

 situation in the circuit. 



The vessels which carry the blood from the heart to the tissues, 

 the arteries, are thick-walled, and contain an abundance of muscular 

 and elastic elements in their walls. The typical medium-sized artery 

 is described as consisting of three coats (Fig. 368) : an intima lined 

 by a continuous layer of flattened endothelial cells, which rest on a 

 well-marked lamina of yellow elastic tissue ; a media composed of 

 unstriated muscular fibres arranged longitudinally and circularly ; 

 and an external coat or adventitia of fibrous tissue, with a number of 

 longitudinal elastic fibres. Near the heart, in the great vessels such 

 as the aorta and its larger branches, there is a preponderance of 

 elastic tissue as compared to the muscular ; and we find in the media 

 alternate layers of muscle fibres and fenestrated elastic membranes. 

 In the smallest arteries, on the other hand, the arterioles, the elastic 

 element entirely disappears, so that the wall consists of muscle fibres, 



