ARTERIES, CONTRACTILITY OF. 



cavity of the cranium, and in the vertebral canal ; the difference depends on 

 the external and middle coats, which in the vessels referred to are thinner 

 than elsewhere. 



The coats of arteries receive small vessels, both arterial and venous, named 

 vasa vasorum, which serve for their nutrition, The little nutrient arteries do 

 not pass immediately from the cavity of the main vessel into its coats, but 

 are derived from branches which arise from the artery (or sometimes from a 

 neighbouring artery), at some distance from the point where they are ulti- 

 mately distributed, and divide into smaller branches within the sheath, and 

 upon the surface of the vessel, before entering its coats. They form a net- 

 work in the tissue of the external coat, from which a few penetrate into the 

 middle coat, and follow the circular course of its fibres ; none have been 

 discovered in the internal coat, unless the observations of Jasche and Arnold 

 are to be trusted, who affirm that they have seen vessels in that situation. 

 Minute venules return the bJood from these nutrient arteries, which, however, 

 they do not closely accompany, and discharge it into the vein or pair of veins 

 which usually run alongside the artery. 



Arteries are generally accompanied by larger or smaller nerves ; and when, 

 in the operation of tying an artery, these happen to be included along with 

 it in the ligature, great pain is experienced, but the vessel itself, when in a 

 healthy condition, is insensible. Nerves are, nevertheless, distributed to the 

 coats of arteries, probably for governing their contractile movements. The 

 nerves come chiefly from the sympathetic, but also from the cerebro-spiual 

 system. They form plexuses round the larger arteries, and run along the 

 smaller branches in form of fine bundles of fibres, which here and there twist 

 round the vessel, and single nerve fibres have been seen closely accompanying 

 minute arteries. The fine branches destined for the artery penetrate to the 

 middle coat, in which they are chiefly distributed. They lay aside their 

 medullary sheath and form a plexus of pale fibres, the finest of which are 

 without nuclei. 



Vital properties. Contractility. Besides the merely mechanical property of elas- 

 ticity, arteries are endowed in a greater or less degree with vital contractility, by 

 means of which they can narrow their calibre. This vital contractility, which has 

 doubtless its seat in the plain muscular tissue of the middle coat, does not cause rapid 

 contractions following in rhythmic succession like those of the heart ; its operation 

 is, on the contrary, slow, and the contraction produced is of long endurance. Its 

 effect, or its tendency, is to contract the area of the arterial tube, and to offer a cer- 

 tain amount of resistance to the distending force of the blood ; and as the contracting 

 vessel will shrink the more, the less the amount of fluid contained in it, the vital con- 

 tractility would thus seem to adjust the capacity of the arterial system to the quan- 

 tity and force of the blood passing through it, bracing up the vessels, as it were, and 

 maintaining them in a constant state of tension. In producing this effect, it co-ope- 

 rates with the elasticity of the arterial tubes, but it can be shown that after that 

 property has reached its limit of operation the vital contraction can go further in 

 narrowing the artery. The vital or muscular contractility of the arteries, then, coun- 

 teracts the distending force of the heart and seems to be in constant operation. Hence 

 it is often named "tonicity," and so far justly ; but at the same time, like the con- 

 tractility of other muscular structures, it can, by the application of various stimuli, 

 be artificially excited to more vivid action than is displayed in this natural tonic or 

 balanced state ; and, on the other hand, it sometimes relaxes more than the habitual 

 degree, and then the vessels yielding to the distending force of the heart become 

 unusually dilated. Such a remission in their contractile force (taking place rather 

 suddenly) is probably the cau*e of the turgescence of the small vessels of the skin 

 which occurs in blushing ; and the arteries of erectile organs are probably affected in 

 the same manner, so as to permit an augmented flow of blood into the veins or 

 venous cavities when erection begins. 



