286 FUNCTIONS OF NUTRITION. 



unstriped muscular fibres, arranged transversely around the vessel ; and 

 an external coat of condensed connective tissue. The principal differ- 

 ence between the larger and smaller arteries is in the structure of 

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

 of muscular fibres. In arteries of medium size it contains both mus- 

 cular and elastic tissue ; while in those of the largest calibre it consists 

 of elastic tissue alone. The large arteries, accordingly, have much 

 elasticity and but little contractility ; while the smaller are contractile, 

 and less elastic. 



Movement of Blood through the Arterial System. The movement 

 of the blood in the arteries is due to the impulse of the ventricular 

 systole. The arterial system may be regarded as a great vascular 

 cavity, subdivided by the successive branching of its vessels, but 

 communicating freely with the heart at one extremity, and with the 

 capillary plexus at the other. At the time of the heart's contraction, 

 the muscular walls of the ventricle close upon its contents ; and as 

 the auriculo-ventricular valves shut back and prevent regurgitation, 

 the blood is forced out from the ventricle through its arterial 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. 

 Under these recurring impulses the blood moves from the heart 

 through the arterial system. 



Arterial Pulse. At each ventricular systole a charge of blood is 

 driven into the arteries, distending them by the additional fluid forced 

 into their cavities. When the ventricle relaxes, its 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 semi-lunar valves, which prevent a backward movement. The blood 

 is accordingly propelled, under the elastic pressure of the arterial walls, 

 into the capillary system. When the arteries, thus partially emptied, 

 have returned to their previous dimensions, they are again distended 

 by another contraction of the heart. This produces, throughout the 

 arterial system, a succession of expansions and reactions, known as the- 

 arterial pulse. 



Since each arterial expansion is produced by a ventricular systole, 

 the pulse, as felt in any superficial artery, is a convenient guide for 

 ascertaining the character of the heart's action. The radial artery at 

 the wrist, owing to its accessible situation, is usually employed for 

 this purpose. Any variation in the frequency, force, or regularity of 

 the heart's movement is indicated by a corresponding modification of 

 the pulse at the wrist. 



The average frequency of the pulse in man is, for the adult male in 

 a state of quiescence, 70 beats per minute. This rate may be accel- 

 erated by muscular exertion. Even the variation of muscular effort 

 between the standing, sitting, and recumbent postures, will make a 

 difference in the frequency of the pulse of from 8 to 10 beats per 

 minute. Age has a marked influence in the same direction. According 



