CHAP. XXVIII.] THE CONTRACTILITY OF ARTERIES. 355 



one of Hunter's experiments, the posterior tibial artery of a dog 

 was laid bare : it was observed, in a snort time, to be so much 

 contracted, " as almost to prevent the blood from passing through 

 it, and when divided the blood only oozed out from the orifice."* 



Mechanical stimulation, applied to a living artery, such as gentle 

 friction with the point of a scalpel or needle, excites in its wall a 

 slow and gradual contraction at the point stimulated, so that it 

 appears constricted at that point. We have, by stimulating an 

 artery in this way, at several points at some distance from each 

 other, produced quite a moniliform appearance of it, causing a series 

 of constrictions separated by portions in which the size of the 

 artery was little altered. Verschuir was among the first to observe 

 the effects of mechanical stimulation upon arteries, and he has 

 recorded the results of his observations in his Inaugural Dissertation 

 de Arteriarum et Venarum vi irritabili, published in 1766; and 

 numerous experiments of a similar kind were performed in this 

 country by our friend, Dr., now Sir Charles Hastings, which are 

 detailed in his treatise on Inflammation of the Mucous Membrane 

 of the Lungs, published in 1820. 



The galvanic stimulus is also capable of producing contractions 

 in arteries, but it requires to be repeatedly renewed before the effect 

 is manifest. The most striking results from the application of this 

 kind of stimulus, were obtained by the Webers, in their experi- 

 ments with the rotatory magneto-electric instrument. The shocks 

 were applied to the small mesenteric arteries of frogs, and a diminu- 

 tion of their diameter to one third was produced, in from four to 

 ten seconds ; and the contraction increased under the continuance 

 of the stimulus, until the calibre of the vessel became from three to 

 six times smaller than at first, so that only a single row of blood 

 corpuscles could pass along it ; at length the vessel became com- 

 pletely closed, and the circulation through it stopped. 



From the combined evidence of anatomy and experiments, then, 

 it can no longer be doubted that arteries possess an inherent con- 

 tractility, in virtue of the presence of unstriped muscular fibres in 

 their tunics. It remains to inquire, in what manner this power 

 influences the circulation in the arterial system. Does it help to 

 propel the blood ? This question may be answered in the negative. 

 The manner in which arterial trunks taper towards their distal 

 extremities, renders it mechanically impossible that the contraction 

 of circular muscular fibres around them would drive the blood 

 onwards unless some valvular apparatus checked its passage back- 

 * Hunter on the Blood, etc., 4to, ed. p. 114. 



