THE SYMPATHETIC SYSTEM. 501 



artery diminishes, the middle coat of the smallest arterial branches 

 being almost exclusively muscular. The fibres of this coat are wrapped 

 round the artery in an annular direction ; producing, when called into 

 activity at any point, a local constriction of the arterial tube. 



Furthermore, it is shown by observation that arteries, like other 

 muscular organs, have the power of contractility. Their contraction 

 in the living animal, under mechanical irritation or galvanic stimulus, 

 has been demonstrated in various observations quoted by Milne Ed- 

 wards,* and corroborated by Tulpianf in more extended experiments 

 on the same subject. The carotid, femoral, hypogastric, interdigital, 

 auricular, and mesenteric arteries, have all been seen to contract when 

 touched with the point of a needle, rubbed with a smooth instrument, 

 or subjected to the galvanic current. There are certain peculiarities 

 in the phenomena thus produced, showing a physiological relationship 

 between the arteries and other organs composed of unstriped muscular 

 fibres. First, the contraction of the artery does not take place imme- 

 diately on the application of the stimulus, but only after a perceptible 

 interval. According to Yulpian, when the electrodes are placed for a 

 few instants in contact with an artery, no effect may be visible on their 

 withdrawal ; but after a short time the vessel diminishes in size, 

 becoming gradually smaller at the point of stimulation, until its 

 calibre is nearly or quite obliterated. It remains in this condition 

 for ten or fifteen seconds, after which it slowly enlarges to its previ- 

 ous size. Secondly, the contractility of the vascular walls under local 

 stimulation is more distinct in the branches than in the trunks of the 

 arteries, and is most pronounced in their smallest ramifications. This 

 corresponds to the anatomical structure of these vessels, in which the 

 muscularity of their middle coat increases with their diminution in size. 



The contraction of an artery under these circumstances has an effect 

 on the local circulation. The diminished calibre of the vessel allows 

 a smaller quantity of blood to pass through it, and thus produces a 

 partial anosmia of the region supplied by its branches. If this region 

 be wanting in collateral inosculations, its change in vascularity may be 

 very marked. All the vascular ramifications beyond the constricted 

 portion of the artery become comparatively bloodless ; and they con- 

 tinue in this condition until the artery relaxes, and again allows the 

 free entrance of blood. While the entire system, therefore, depends 

 on the heart as an organ of impulsion for the circulation in general, 

 each artery controls, for its own special region, the quantity of blood 

 admitted to the capillaries and veins. 



Rhythmical Contraction of Arteries in Particular Parts. If the 

 ear of a white rabbit be held for a few moments against the light it 

 will be seen that its blood-vessels change their appearance from time to 

 time, and that this change occurs with a certain regularity. The cen- 



* Le9ons sur la Physiologie et T Anatomie Comparee. Paris, 1859, tome iv., p. 207. 

 f Leyons sur 1'Appareil Vasomoteur. Paris, 1875, tome i., p. 43. 



