658 SPECIAL PHYSIOLOGY. 



Motion of the Blood through the Arteries, and Influence of those 

 Vessels on the Circulation. 



The phenomena of the circulation of the blood thro'ugh the arteries, 

 have been studied exclusively in the systemic arterial vessels; for the 

 pulmonary arteries and their branches are removed from direct obser- 

 vation and experiment. The structure and distribution of the arteries, 

 the properties of their coats, and their mode of subdivision and anas- 

 tomoses, already described (pp. 25 and 54), have important influences 

 on the motion of the blood through them. The very smooth, glassy sur- 

 face of the internal coat serves, like that of the endocardium of the 

 heart, to diminish the friction between the blood and the sides of the 

 bloodvessels. 



The remarkable physical property of elasticity, possessed by the 

 middle arterial coat, is of extreme importance ; it exists in a more 

 striking degree in the larger arteries, especially in those near to the 

 heart ; it is manifested not only on stretching the vessel in a lateral, 

 but also in a longitudinal direction. Two purposes are served by this 

 elasticity ; first, it protects the arteries against the force of the heart, 

 to which they yield, instead of offering a rigid resistance ; and, secondly, 

 it enables them to recoil, after they have thus yielded, and to react 

 upon the column of blood within them. It is this recoil which gradu- 

 ally converts the intermittent force of the heart into a continuous pres- 

 sure in the small vessels. Moreover, the elasticity of the arteries 

 enables them to bear occasional increase in the quantity of blood forced 

 into them from the ventricles, as in conditions of excitement ; or a more 

 permanent addition to the normal quantity, as in plethora. Lastly, 

 it prevents their compression by the ordinary muscular movements, 

 and permits them to bend and elongate, and so to accommodate them- 

 selves to changes of position in the trunk and limbs. 



The vital contractility of the involuntary muscular arterial walls, is 

 of equal importance. Contrary to what is the case with their elasticity, 

 this contractility is feebly manifested by the larger arteries, but is very 

 active in the smaller ones. This property of the arteries is shown by 

 their slow contraction after death, owing to which, when no longer dis- 

 tended by the force of the heart, they contract, and are usually emptied 

 of blood; also by their contraction under the influence of cold, heat, 

 and mechanical, chemical and electrical stimuli, applied either to them- 

 selves, or to their nerves. Like the contractility of the other muscular 

 fibres of organic life, that of the arteries is slow in its manifestation. 

 Different stimuli, however, act differently in exciting it. Some are 

 said to cause slow contraction, and others a more rapid contraction, 

 with subsequent slow return to the natural state; some speedily produce 

 marked dilatation, and others, a dilatation, followed slowly by a per- 

 .sistent condition of contraction. The stronger the stimulus, the more 

 likely is it to produce this latter effect. The tonic contraction of an 

 artery is powerfully excited by cold; whilst warmth relaxes it; but a 

 caustic heat causes the most durable contraction, which may, in part, 

 explain the effect of the actual cautery in arresting hemorrhage. John 



