CIRCULATION IN THE AETERIES. 65 



property of elasticity ; and the intermittent impulse may be said to be pro- 

 gressively absorbed by the elastic walls of the vessels. This modification of 

 the impulse of the heart has great physiological importance ; for it is evi- 

 dently essential that the current of blood, as it flows into the delicate capil- 

 lary vessels, should not be alternately intermitted and impelled with the full 

 power of the ventricle. 



The elasticity of the arteries favors the flow of blood toward the capillaries 

 by a mechanism that is easily understood. The blood discharged from the 

 heart distends the elastic vessel, which reacts, after the distending force 

 ceases to operate, and compresses its fluid contents. This reaction would 

 have the effect of forcing the blood in two directions, were it not for closure 

 of the valves, which renders regurgitation into the heart impossible. The 

 influence, then, can be exerted only in the direction of the periphery. It is 

 evident, therefore, that in vessels removed a sufficient distance from the heart, 

 the force exerted on the blood by the reaction of the elastic walls is compe- 

 tent to produce a very considerable current during the intervals of the heart's 

 action. 



Contractility of the Arteries. The medium-sized and smallest arteries 

 contain non-striated muscular fibres ; and it has been shown that as a con- 

 sequence of the condition of these fibres, the vessels undergo considerable 

 variations in their caliber. These changes in the size of the arteries can be 

 produced by stimulation or section of the vaso-motor nerves. If the sympa- 

 thetic be divided in the neck of a rabbit, the arteries of the ear on that side 

 soon become dilated. If the divided, extremity of the nerve be stimulated, 

 the vessels contract and may become smaller than on the opposite side. 

 These experiments demonstrate the contractile properties of the small arteries 

 and give an idea how the supply of blood to any particular part may be regu- 

 lated. The contractility of the arteries has great physiological importance. 

 As their office is simply to supply blood to the various tissues and organs, it 

 is evident that when the vessels going to any particular part are dilated, the 

 supply of blood is necessarily increased. This is particularly well marked in 

 the glands, which, during the intervals of secretion, receive a comparatively 

 small quantity of blood. 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 emo- 

 tions are examples of the same kind of action. 



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 erroneous ; and it is hardly necessary to repeat the statement that 

 the cause of the arterial circulation is the force of the left ventricle. It has 

 been observed, however, that the arteries in the ear and certain other parts 

 in the rabbit undergo rhythmical contractions and dilatationSj these occur- 

 ring ten or twelve times per minute (Schiff, Loven, Vulpian) ; but these 

 movements are not to be regarded as a contributing force in the production 

 of the circulation. It is evident, on the othei hand, that the elasticity of the 

 arteries must actually assist the circulation. The resiliency of the vessels is 



