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tinued on different sides into somewhat larger tubes, with thicker walls, which are the smallest arteries and veins.

The mere fact that the walls of these vessels are thicker than those of the capillaries constitutes an important difference between the capillaries and the small arteries and veins ; for the walls of the latter are thus rendered far less permeable to fluids, and that thorough irrigation of the tissues, which is effected by the capillaries, cannot be performed by them.

The most important diffe nce between th vessels and the capillaries, however, lies in the circumstance that their walls are not only thicker, but also more complex, being composed of several coats, one, at least, of which is muscular. The number, arrangement, and even nature of these coats differ according to the size of the vessels, and are not the same in the veins as in the arteries, though the smallest veins and arteries tend to resemble each other.

If we take one of the smallest arteries, we find, first, a very delicate lining of cells constituting a sort of epithelium (Fig. 3 B, a). Outside this (separated from it by a structureless membrane, Fig. 3 B, 6) comes the muscular coat of the kind called plain or non-striated muscle (see Lesson XII.), made up of flattened spindle-shape bands or fibres which are wrapped round the vessel (Fig. 3 B, c).

Outside the muscular coat is a sheath of fibrous or connective tissue (Fig. 3 B, f).

In the smallest arteries there is but a single layer of these muscular fibres encircling the vessel like a series of rings ; but in the larger arteries there are several layers of circular muscular fibres variously bound together with fibrous and elastic tissue, though as the vessels get larger the quantity of muscular tissue in them gets relatively less.

Now these plain muscular fibres possess that same power of contraction, or shortening in the long, and broadening in the narrow, directions which, as was stated in the preceding Lesson, is the special property of muscular tissue. And when they exercise this power, they, of course, narrow the calibre of the vessel, just as squeezing it in any other way would do; and this contraction may go so far as, in some cases, to reduce the cavity of the vessel almost to nothing, and to render it practically impervious.

The state of contraction of these muscles of the small arteries and veins is regulated, like that of other muscles, by their nerves ; or, in other words, the nerves supplied to the vessels determine whether the passage through these tubes should be wide and free, or narrow and obstructed. Thus while the small arteries and veins lose the function, which the capillaries possess, of directly irrigating the tissues by transudation, they gain that of regulating the supply of fluid to the irrigators, or capillaries themselves. The contraction, or dilatation, of the arteries which supply a set of capillaries, comes to the same result as lowering or raising the sluice-gates of a system of irrigation-canals.

3. The smaller arteries and veins severally unite into, or are branches of, larger arterial or venous trunks, which again spring from or unite into still larger ones, and these, at length, communicate by a few principal arterial and venous trunks.with the heart.

The smallest arteries and veins, as we have seen, are similar in structure, but the larger arteries and veins differ widely; for the larger arteries have walls so thick and stout that they do not sink together when empty; and this thickness and stoutness arises from the circumstance that not only is the muscular coat very thick, but that, in addition, and more especially, several layers of a highly elastic, strong, fibrous substance become mixed up with the muscular layers. Thus, when a large artery is pulled out and let go, it stretches and returns to its primitive dimensions almost like a piece of india-rubber.

The larger veins, on the other hand, contain but little of either elastic or muscular tissue. Hence, their walls are thin, and they collapse when empty.

This is one great difference between the larger arteries and the veins; the other is the presence of what are termed valves in a great many of the veins, especially in those which lie in muscular parts of the body. They are absent in the largest trunks, and in the smallest branches, and in all the divisions of the portal, pulmonary, and cerebral veins.

4. These valves are pouch-like folds of the inner wall of the vein. The bottom of the pouch is turned towards those capillaries from which the vein springs. The free edge of the pouch is directed the other way, or towards the heart. The action of these pouches is to impede the passage of any fluid from the heart towards the capillaries, while they do not interfere with fluid passing in the opposite direction (Fig. 4). The working of some of these valves may be very easily demonstrated in the living body. When the arm is bared, blue veins may be seen running from the hand, under the skin, to the upper arm. The diameter of these veins is pretty even, and diminishes regularly towards the hand, so long as the current of the blood, which is running in them, from the hand to the upper arm, is uninterrupted.

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Fig. 4.-DIAGRAMMATIC SECTIONS of Veins with VALVES.

In the upper, the blood is supposed to be flowing in the direction of the arrow, towards the heart; in the lower, the reverse way. C, capillary side ; H, heart side.

But if a finger be pressed upon the upper part of one of these veins, and then passed downwards along it, so as to drive the blood which it contains backwards, sundry swellings, like little knots, will suddenly make their appearance at several points in the length of the vein, where nothing of the kind was visible before. These swellings are simply dilatations of the wall of the vein, caused by the pressure of the blood on that wall, above a valve which opposes its backward progress. The moment the backward impulse ceases the blood flows on again ; the valve, swinging back towards the wall of the vein, affords no obstacle to its progress, and the distension caused by its pressure disappears (Fig. 4).

The only arteries which possess valves are the primary trunks—the aorta and pulmonary artery—which spring

from the heart, and they will be best 9

considered with the latter organ.

5. Besides the capillary network and the trunks connected with it, which constitute the blood-vascular system, all parts of the body which possess blood capillaries — except the brain and spinal cord, the eyeball, the gristles, tendons, and perhaps the bones also contain another set of what are termed lymphatic capilla

ries, mixed up with those of the blood9

vascular system, but not directly communicating with them, and, in addition, differing from the blood capillaries in being connected with larger vessels of only one kind. That is to say, they open only into trunks which carry fluid away from them, there being no large vessels which bring anything to them.

These trunks further resemble the small veins in being abundantly provided with valves which freely allow of the passage of liquid from the lym

phatic capillaries, but obstruct the Fig. 5.—The LYMPHA- flow' of anything the other way. But TICS OF THE Front of the lymphatic trunks differ from the

veins, in that they do not rapidly unite s Lymphatic glands, or ganglia, as they are

into larger and larger trunks, which sometimes called. These present a contin

ually increasing caliganglia are not to be þre, and allow of a flow without intervous ganglia. ruption to the heart.

On the contrary, remaining nearly of the same size, they, at intervals, enter and ramify in rounded bodies called lymphatic glands, whence new lymphatic trunks arise (Fig. 5). În these glands the

1 It is probable that these exceptions are apparent rather than real, but the question is not yet satisfactorily decided.

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Fig. 6.—THE THORACIC Duct. The Thoracic Duct occupies the middle of the figure. It lies upon the spinal column, at the sides of which are seen portions of the ribs (1)." a, the receptacle of the chyle; b, the trunk of the thoracic duct, opening

at c into the junction of the left jugular (f) and subclavian (g) veins as they unite into the left innominate vein, which has been cut across to show the thoracic duct running behind it; d, lymphatic glands placed in the lumbar regions; h, the superior vena cava formed by the junction of the right and left innominate veins.

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