MOTION OF THE BLOOD IN THE ARTERIES. 559 



performed some ingenious experiments, which prove that the force required, 

 to propel fluid through a tube, whose sides are yielding, is very much greater 

 than that which will carry it through a tube of even smaller size, with rigid 

 parietes; consequently, a loss of tonicity in the blood-vessels retards the flow 

 of blood through them ; whilst an increase hastens it. The Tonicity of the 

 arteries differs from their ordinary Contractility, in being augmented by cold, 

 and diminished by warmth. Hence cold and heat are two most valuable 

 remedial agents, when this property is deficient or in excess. 



732. It is still to be inquired, in what manner the Contractility of the Arte- 

 ries is to be regarded as influencing the flow of Blood through them. It is at 

 once evident, that any general contraction of the arterial tubes would have 

 rather the effect of opposing, than of assisting the flow ; but if the fibrous 

 coat of the Arteries is in some degree disposed to the alternate contraction and 

 relaxation, which are so remarkable in the Heart, they may exert a force which 

 shall be supplementary to that of the Heart's impulse, relaxing to receive 

 the blood from it, and contracting upon their contents, with a power superior 

 to that by which they were distended. It is difficult to say whether or not 

 this be the case ; though there would certainly appear some evidence in favour 

 of the supposition. The loss of the Heart's power over the currents of blood, 

 in proportion to their degree of subdivision, occasioned by the increased fric- 

 tion to which they will be subjected, would seem to require some compensat- 

 ing power, in order that the perfect equality of pressure may be obtained 

 which has been spoken of as existing in all parts of the arterial system. In 

 no other way than this can the fibrous coat of the Arteries be regarded as 

 having any propulsive power over their contents ; except by a peristaltic or 

 vermicular movement, resembling that which takes place in the alimentary 

 canal ; and of such there is no evidence whatever. A very important use 

 may be assigned to this muscular coat, which has been generally overlooked 

 by Physiologists, that of regulating the diameter of the tubes, in accord- 

 ance with the quantity of blood to be conducted through them to any part ; 

 which will depend upon its peculiar circumstances at the time. Such local 

 changes are continually to be observed, in the various phases of normal life, 

 as well as in diseased states ; and they will be found to be constantly in har- 

 mony with the particular condition of the processes of Nutrition, Secretion, 

 &c., to which the Capillary circulation ministers. Of this kind are the en- 

 largement of the trunks of the Uterine and Mammary arteries, at the epochs 

 of pregnancy and lactation ; the enlargement and strongly-increased pulsa- 

 tion of the Radial artery, when there is any active inflammation in the thumb ; 

 the enormous diameter which the Spermatic artery will attain, when the 

 testicle is greatly increased in size by diseased action; and many other simi- 

 lar phenomena. In such cases, it cannot be the action of the Heart that in- 

 creases the calibre of the vessels ; since this is commonly unaltered, and is 

 itself unable, as we have just seen, even to maintain their permeability. It 

 must, therefore, be by a power inherent in themselves, that their dilatation is 

 effected. The minute distribution of the Sympathetic nerve upon the walls 

 of the arteries, the known power which this has of producing contractions, 

 alike in their fibrous coat, and in the muscular tunic of the intestinal canal, 

 and various phenomena, which indicate the power of certain states of mind 

 over the dimensions of the arteries, in particular parts of the body at least, 

 render it highly probable that the calibre of the arteries is regulated in no in- 

 considerable degree through its intervention.* The permanent dilatation, 

 however, which is seen in the arteries supplying parts that are undergoing 



" For Anatomical evidence to this effect, see Henle on the Contractility of the Blood- 

 vessels, in Casper's Wochenschrift, May 1840, and Brit, and For. Med. Rev., vol. x. p. 551. 



