The arterial tubes arc composed of several layers, partly membranous and partly muscular. They are all furnished with a peripheral or external coat, which is derived chiefly from the cellular substance by which they are every where surrounded. This cellular sub stance is sometimes so dense and firm, as to form a ca nal or sheath, within which the artery is contained. The elasticity of the arteries seems to reside chiefly in their external coat. Some of the larger trunks derive their additional coat from the membrane that lines the cavity where they are lodged, from the peritoneum, the pleura, pericardium, Jura mater, &c. The arteries are lined on their central surface with a membrane that is very thin, fine, smooth and transparent, though at the same time dense and strong. Between these lies the proper mus cular or fibrous coat, which is peculiar to the arterial system. This coat is very evident in the larger trunks and branches ; but as the arteties diminish in size, it becomes less apparent.' The lihres of which this coat is composed, are disposed in a circular dire( tion, form ing arches round every part of the cylinder, but not per fect circles. The whole muscular coat appears of a pale reddish, or rather, according to Maio., of a yel lo•ish colour, and 'here it is thickest, the artery is most opaque. It is this coat that gives to the arteries their contractile power. All these coats are connected together by cellular substance. They are supplied with nourishment by extremely small arteries, called by anatomists VItO! VasOrilln, and they are furnished tt with absorbents, and apparently with nerves.
The minutest branches in which most of the arteries terminate, are called capillary vessels, limn their ex treme slenderness. Where these minutest branches terminate, another set of vessels commences, which re ceive the blood from the arteries through the capilla ries. These arc the veins, and these vessels we shall presently consider. The capillary vessels, as lying be tween the arteries and veins, may be said to belong to both systems, and will be more properly considered by themselves.
It is not only in the capillary vessels that the arteries terminate. Some of them open into various cavities by small vessels that are termed exhalants; others pour their contents into certain complex organs called glands, destined to produce sonic change on the blood w hich they receive, while a few convey blood into spongy cel lular parts, front which it is main taken up, nearly un changed, by other vessels.
The blood contained in the principal trunks and branches of the arteries, when these are viewed through a microscope, appears of a red colour; but in tile smaller ramifications, and espci ially in the capillaries, it is nearly colourless. We are not, however, to suppose, that the coloured blood dues not flow through the ca pillaries from the arteries into the veins ; but from the extreme slenderness of the stream in these minute ves sels, its colour is much less obvious ; just as a small luantity of red wine in the bottom of a glass appears of a touch lighter tinge than a considerable quantity in the same vessel.
The arteries in young people arc proportionally larger, and heat more strongly than in persons of an advanced age. In very old people many of the 'Miitel' arterial brunches become import ious, and the diameter of others is greatly diminished.
The names, origins, distribution and connection of the principal arteries of the human body will be given presently in a tabular !bros.
Of the 1"( Tito veins are elastic tubes, which carry hack to the auricles of the heart that blood which had been dist•i buted by the arteries to every part of the body. They arc generally huger than the corresponding arteries, and in the living body are easily distinguished from these by their want of pulsation. Many of them are confined to the surface tit the body, r [inning iu very oh % ions ramifications la low the skin, where they dis tinguished by their blue ( and hitwccu the mils ( les, hut two braliChl S a.1110St Uhl Lir, :Ler 1.11C deep-seated arteries. Ilunce their absolute riumbei. is greater than that of the al tern's, exci pt, perhaps, in the lungs w here the number and relative size ol the arterial and 'emus branches are nearly alike. The veins are cylindrical like the ark ries, hut their sides are thinner, weaker, and less elastic, w lichee diet do not preserve the cylindrical form of their cavities NV liC1 cut across, or w hen a great part of their content, is ab stracted. Their direction and ramification are similar to that Of arteries. 'filet also make fueyient awasto muses, or their branches freely communicate with eat It other. They are also every w here surroulided a ith cellular texture, but this is schloni so as deli Bich forms the. sheathe, of the arteries.
Tile veins are composed, like the arteries, or tin, coats, on outer cellular membrane, an internal delicate membrane, and an intermediate fibrous coat ; but dies. coats differ in several respects from those or the arte ries. The cellular coat is looser, and less elastic. The fibrous coat i3 not composed of circular fibres like that of the arteries, but chiefly of longitudinal libri s parallel to each other, and. orti, with considerable interNak be tween them. These longitudinal fibres are said by Ilichat to he more appareot in the superficial than id the deep-scat«I veins, and are by him considered as essentially distinct in their nature and properties, from the circular fibres that compime the muscular coat or the arteries. They are not elastic, are of a ,oll texture, hut extremely extensible, and not easily broken. See Iffeloai ..burionric (:rvJv'ab•, tom. ii. p. 39.), -to 4. Some anatomists are of opinion that these fibres are muscular ; but this is by no means certain, and from their direction they cannot have the effect so inn\ ersally attributed to the circular fibres of the arteries, of assisting the circu lation of the blood. In several of the veins, the internal membrane is smooth and continuous ; hut in a great number, especially those of the extremities, this mem brane is furnished with semiluna• folds projecting- w hhIll the cavity of the tube, and forming valves that are con cave towards the trunks, and convex towards the small branches of the veins. These VcS readily allow the blood to pass from the branches into the trunks towards the heart, but completely obstruct its passage in the con t•ary direction. \\There the valves are seated, small protuberances, like knots, may sometimes be observed On the outside, arising, as is alleged, from the insinua tion of the blood between the concave side of the Val yes, and the adjacent part of the sides of the veins.