The Philosophy of Health; Volume 1 (of 2) / or, an exposition of the physical and mental constitution of man

by Southwood Smith

Portion of an artery, showing the several coats of which
it is composed separated from each other. 1. The internal
or serous coat; 2. the middle or fibrous coat; 3. the external
or cellular coat.

270. The middle or the fibrous tunic is composed of yellowish flattened fibres which pass in an oblique direction around the calibre of the vessel, forming segments of circles, which, uniting, produce complete rings (fig. CXVIII. 2). This tunic is thick, consisting of several layers of fibres which it is easy to peel off in succession. They form a firm, solid, elastic, but, at the same time, brittle membrane.

271. The inner tunic, thin, colourless, nearly transparent, and perfectly smooth, is moistened by a serous fluid, and is thence called the serous coat (fig. CXVIII. 1). To the naked eye it presents no appearance of fibres, yet notwithstanding its extreme delicacy, it is so strong that, after the other coats of the artery have been entirely removed in a living animal, it is capable of resisting the impetus of the circulation, and of preventing the dilatation of the artery. The arteries themselves are supplied with arteries, vessels that nourish their tissues, and which are sent to them from neighbouring branches, seldom or never from the vessel itself to which they are distributed. Each individual part of an artery is supplied by its own appropriate vessels, which form but few communications above and below, so that if care be not taken in surgical operations to disturb these nutrient arteries very little, the vessel will perish for want of sustenance.

272. The vein, the tube that carries back the blood to the heart, is composed of the same number of tunics as the artery, which, with the exception of the middle, are essentially the same in structure, but they are all much thinner. The external tunic consists of a less dense and strong cellular membrane; the middle tunic, instead of being formed of elastic rings, is composed of soft and yielding fibres, disposed in a longitudinal direction; while the inner coat, which is still more delicate than that of the artery, is arranged in a peculiar manner. The inner coat of most veins, at slight intervals, is formed into folds (fig. CXX. 5), one margin of which is firmly adherent to the circumference of the vessel, while the other margin is free and turned in the direction of the heart. These membranous folds are termed valves. In all veins the diameter of which is less than a line the valves are single; in most veins of greater magnitude they are placed in pairs, while in some of the larger trunks they are triple, and in a few instances quadruple, and even quintuple. The veins, like the arteries, are supplied with nutrient vessels and nerves.

273. All the arteries of the body proceed from the two trunks already described; that connected with the pulmonic circle, the pulmonary artery, and that connected with the systemic circle, the aorta. These vessels, as they go out from the heart and proceed to their ultimate termination, are arborescent, that is, they successively increase in number and diminish in size, like the branches of a tree going off from the trunk (fig. CXIX. 1, 2, 3). Each trunk usually ends by dividing into two or more branches (fig. CXIX. 1, 2), the combined area of which is always greater than that of the trunk from which they spring, in the proportion of about one and a half to one. As the branch proceeds to its ultimate termination it divides and subdivides, until at length the vessel becomes so minute, that it can no longer be distinguished by the eye. These ultimate branches are called capillary vessels, from their hair-like smallness (fig. CXIX. 4); but this term does not adequately express their minuteness. It has been stated (234) that the red particle of the blood, at the medium calculation, is not more than the three-thousandth part of an inch in diameter; yet vast numbers of the capillary vessels are so small that they are incapable of admitting one of these particles, and receive only the colourless portion of the blood.

View of the manner in which an artery divides and subdivides
into its ultimate branches. 1. Trunk of the artery;
2. large branches into which it subdivides; 3. small
branches, successively becoming smaller and smaller until
they terminate in 4. the capillary branches.

274. Every portion of an artery, by reason of the elasticity of its coats, preserves nearly a cylindrical form, and as the area of the branches is greater than that of the trunks, the blood, in proceeding from the heart to the capillaries, though passing through a series of descending cylinders, is really flowing through an enlarging space.

275. The disposition of the veins, like that of the arteries, is arborescent, but in an inverse order; for the course of the veins is from capillary vessels to visible branches, and from visible branches to large trunks (fig. CXX. 1, 2, 3). In every part of the body where the capillary arteries terminate the capillary veins begin, and the branches uniting to form trunks, and the small to form large trunks, and the trunks always advancing towards the heart, and always increasing in magnitude as they approach it, form at length the two veins which it has been stated (258) return all the blood of the body to the right auricle of the heart.