the middle of the thickness of the wall of the vessel; inside it, and lining the cavity of the vessel, is a layer of very delicate, elongated, epithelial cells (Fig. 5, A; Fig. 6, c). Outside the muscular layer is a sheath of fibrous tissue (a, Fig. 6). The muscular fibres themselves are flattened, spindleshaped bands, each with an elongated rod-like nucleus in the middle (Fig. 5, B). When these

A. Epithelial cells of the arteries. a, The nucleus.

B. Muscular fibres of the arteries: the middle one having been treated with acetic acid, shows more distinctly the nucleus a.

Magnified about 350 diameters.

fibres exercise that 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), 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 shall be free and wide, or narrow and obstructed. Thus while the small arteries and veins lose the function of directly irrigating the tissues which the capillaries possess, 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.

A. A small artery.

FIG. 6.

B. A small vein, both treated with acetic acid: a, fibrous coat; b, nuclei of the muscular coat; c, nuclei of the epithelial coat,

Magnified about 300 diameters.

3. The smaller arteries and veins severally unite into, or are branches of, larger arterial or venous trunks, which again spring from 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, a strong coat of highly elastic fibrous substance is developed outside the muscular layer. 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.

4. These valves are pouch-like folds of the inner wall of the vein. The bottom of the pouch is turned towards those capillaries into which the vein opens. 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. 7). 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

H

H

FIG. 7.-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.

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.

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. 7).

The only arteries which possess valves are the primary trunks-the aorta and pulmonary arterywhich spring from the heart, and they will be best considered with the latter organ.

FIG. 8.-THE LYMPHATICS OF THE FRONT OF THE RIGHT ARM.

g Lymphatic glands, or ganglia, as they are sometimes called. These ganglia are not to be confounded with nervous ganglia.

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

D.