298 



THE CIRCULATION 



proper; and the veins are passive completers of the circulation round to 

 the heart after the blood's work is done. 



The Physiology of the Arteries. The structure of the blood-vessels 

 will be found described in the text-books of anatomy and of histology. 

 The structure of each sort of vessel is immediately dependent on its 

 functions and vice versa. The arterial walls are very elastic and strong, 

 and contractile by means of the thick layers of unstriated muscle-cells 

 contained in them. 



In discussing the causes of the circulation we have already seen in 

 what way the passive elasticity of the arteries serves the distribution of 

 blood. It acts as a propelling force during the five-eighths of the time 

 when the ventricles are not contracting. The pressure is purely a passive 



recoil of elastic tissues and not 



FlG - 158 an active muscular movement. 



The elasticity is of use furthermore 

 in allowing of the distention of the 

 arteries to accommodate the sud- 

 den influx of blood at each heart- 

 beat. It is the uprise of the 

 arterial wall during this influx 

 which gives the pulse, long an 

 important practical matter in 

 medical art and science. 



THE PULSE may be felt with the 

 finger over any artery not too 

 small or too deeply hidden in the 

 body. The vessel most often em- 

 ployed for observation of the qual- 

 ities of the pulse is the radial 

 artery in the wrist, although the 

 temporal is often used just anterior 

 to the tragus of the ear, especially 

 in case of children. The caro- 

 tids at either side of and just 



below the larynx are sometimes convenient, but they indicate less than 

 the others because of the soft tissues, rather than bone, behind them. 

 Wherever it be felt, the essential element of the pulse is an uprise of the 

 arterial wall toward the finger as the tube distends with the blood pushed 

 onward from the ventricle. The trained finger readily distinguishes 

 the gradual though quick hardening of the arterial cylinder and its more 

 gradual softening again, at each heart-beat. The conditions are such 

 that the pulse gives information not only concerning the vigor of the 

 heart, the quickness of its systole, the pulse-rate, and whether the 

 valves are working properly or not, but also to the trained observer the 

 scarcely less important information as to the relative elasticity or rigidity 

 of the arterial wall, the tonal size of the artery, and the resistance periph- 

 eral to it. These seven relations, and others of less practical impor- 



Cross-section of medium sized art_ry: a, endo- 

 thelium cells lining the lumen; b, internal elastic 

 membrane; c, subendothelial connective tissue; 

 d, muscle-cells of media; e, connective tissue of 

 adventitia; /, vasa vasorum. (Bates.) 



