CONTRACTILITY OF THE ARTERIES. 



69 



out by the following simple and conclusive experiment of Marey : He connected two 

 tubes of equal size, one of rubber and the other of glass, with the stop-cock of a large 

 vase filled with water. The elastic tube was provided with a valve near the stop-cock, 

 which prevented the reflux of fluid, and both were fitted with tips of equal caliber. 

 When, by alternately opening and closing the stop-cock, water was allowed to flow 

 into these tubes in an intermittent stream, it was found that a greater quantity was 



Y <EEfe 



FIG. 21. Apparatus for showing the, action of the elasticity of the, arteries. (Marey.) 

 V, vessel of water; E, stop-cock ; T, double tube ; S, valve; a, a, glass tube; &, 6, rubber tube. 



discharged by the elastic tube ; but an equal quantity was discharged by both tubes 

 when the stop-cock was left open and the fluid allowed to pass in a continuous stream. 

 This simple experiment shows that not only does the elasticity of the arteries convert the 

 intermittent current in the largest vessels into a current more and more nearly continuous 

 as we approach the periphery, but that when reflux is prevented, as it is by the semilunar 

 valves, the resiliency of the arteries assists the circulation. 



Contractility of the Arteries. It is a well-established anatomical fact that the 

 medium-sized and smallest arteries contain contractile elements ; and it is also a fact, 

 proven by actual experiment, that, as a consequence of the condition of these fibres, 

 the vessels undergo considerable variation in their caliber. The opinions of the older 

 physiologists on this question have only an historical interest and will not, therefore, 

 be discussed. Among the more recent investigations on this subject, we have the experi- 

 ments of Cl. Bernard and of Schiff, which have been repeatedly confirmed, showing that, 

 through the nervous system, the muscular coats of arteries may be readily made to con- 

 tract or become relaxed. If the sympathetic be divided in the neck of a rabbit, in a very 

 few minutes the arteries of the ear on that side are notably dilated. If the divided ex- 

 tremity of the nerve be galvanized, the vessels soon take on contraction and may become 

 smaller than on the opposite side. These experiments demonstrate, in the most conclu- 

 sive manner, the contractile properties of the small arteries and give us an idea how the 

 supply of blood to any particular part may be regulated. The vessels may be most ef- 

 fectually excited through the nervous system ; and it is on account of the difficulty in 

 producing marked results by direct irritation, that the older physiologists were divided 

 on the subject of their "irritability." 



The contractility of the arteries has great physiological importance. As their func- 

 tion 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 in- 

 creased. This is particularly well marked in the glands, which, during the intervals of 

 secretion, receive a comparatively small quantity of blood. Bernard has shown that gal- 

 vanization of what he calls the motor nerve of a gland dilates the vessels, largely increases 



