70 CIRCULATION OF THE BLOOD. 



the supply of blood, and induces secretion ; while galvanization of the sympathetic fila- 

 ments contracts the vessels, diminishes the supply of blood, and arrests secretion. The 

 pallor of parts exposed to cold, and the flush produced by heat, are due, on the one hand, 

 to contraction, and on the other, to dilatation of the small arteries. Pallor and blushing 

 from mental emotions are examples of the same kind of action. 



The ulterior effects on nutrition, which result from dilatation of the vessels of a part, 

 are of great interest. When the supply of blood is much increased, as in section of the 

 sympathetic in the neck, nutrition is exaggerated, and the temperature of the part is 

 raised beyond that of the rest of the body. 



The idea, which at one time obtained, that the arteries were the seat of rhythmical 

 contractions which had a favorable influence on the current of blood is entirely erro- 

 neous. 1 It is hardly necessary to repeat that the prime cause of the arterial circulation 

 is the force of the left ventricle. We have seen that the elasticity of the arteries pro- 

 duces a flow during the intervals of the heart's action, and the question now arises 

 whether the force thus exerted be simply a return of the force required to expand the 

 vessels, which has been borrowed, as it were, from the heart, or something superadded 

 to the force of the heart. The experiment of Marey, already alluded to, settles this ques- 

 tion. When water was forced in an intermittent current into two tubes, one elastic and 

 the other inelastic but discharging by openings of equal size, by far the greater quantity 

 was discharged by the elastic tube. A little reflection will show how the action of the 

 elastic arteries must actually assist the circulation. The resiliency of the vessel is con- 

 tinually pressing their contents toward the periphery, as regurgitation is rendered impos- 

 sible by the closure of the semilunar valves. The dilatation of the vessels with each sys- 

 tole of course admits an increased quantity of blood ; and it has been experimentally 

 demonstrated that the same intermittent force exerted on an inelastic tube will discharge 

 a less quantity of liquid from an opening of equal caliber. 



Superadded, then, to the direct action of the heart, we must recognize, as a cause in- 

 fluencing the flow of blood in the arteries, the resiliency of the vessels, especially of those 

 of large size, this force being derived originally from the heart. Thus it will be seen that 

 the arteries are constantly kept distended with blood by the heart ; and, by virtue of 

 their elasticity and the progressive increase in the capacity of this system as they 

 branch, the powerful contractions of the central organ only serve to keep up an 

 equable current in the capillaries. The small vessels, by virtue of their contractile 

 walls, regulate the distribution of the blood, acting as the guards or sentinels of 

 the process of nutrition, and, in fact, of all the numerous functions in which the blood 

 is concerned. 



Locomotion of the Arteries and Production of the Pulse. At each contraction of the 

 heart, the arteries are increased in length and many of them undergo a considerable loco- 

 motion. This may be readily observed in vessels which are tortuous in their course and 

 is frequently very marked in the temporal artery in old persons. The elongation may 

 also be seen if we watch attentively the point where an artery bifurcates, as at the divis- 

 ion of the common carotid. It is simply the mechanical effect of sudden distention, 

 which, while it increases the caliber of the vessel, causes an elongation even more 

 marked. 



The finger placed over an exposed artery or one which lies near the surface expe- 

 riences a sensation at every beat of the heart as though the vessel were striking against 

 it. This has long been observed and is called the pulse. Ordinarily it is appreciated 

 when the current of blood is subjected to a certain amount of obstruction, as in the radial, 



1 Schiff has noticed rhythmical contractions in the superficial arteries of the ear in the rabbit and in some other ani- 

 mals ; but this phenomenon is exceptional, and the movements do not appear to favor the current of blood. The 

 recent experiments of Dr. J. J. Mason, of New York, show conclusively, to our mind at least, that there is not a peri- 

 staitic action of the muscular coats of the small arteries, capable of assisting the circulation. This view, however, is 

 opposed to the ideas of Legros and Onimus and of some other physiologists. 



