250 CIRCULATION. 



nous 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 fully established 

 anatomical fact that the medium-sized and smallest arteries 

 contain contractile or muscular 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 physi- 

 ologists 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 experiments of Cl. 

 Bernard and Schiif, which have been repeatedly confirmed, 

 showing that through the nervous system the muscular coats 

 of arteries may be readily made to contract or become re- 

 laxed. If the sympathetic be divided in the neck of a rab- 

 bit, in a very few minutes the arteries of the ear on that side 

 are notably dilated. If the divided extremity of the nerve 

 be feebly galvanized, the vessels soon take on contraction, and 

 may become smaller than on the opposite side. These expe- 

 riments demonstrate, in the most conclusive manner, the con- 

 tractile- properties of the small arteries, and give us an idea 

 how the supply of blood to any particular part may be regu- 

 lated. The vessels may be most effectually 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 " irrita- 

 bility." 



The contractility of the arteries has great physiological 

 importance. As their function 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 impor- 

 tant in the glands, which, during the intervals of secretion, 

 receive a comparatively small quantity of blood. Bernard 



