MIDDLE ARTERIAL COAT.

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You will next remember that the outer coat of an artery, though only the second thickest of the three composing it, is much the toughest, and hence when a ligature is tied tightly round one of these vessels, the middle and innermost coats will be cut through, while the outer resists, and maintains its integrity.

The middle coat of an artery, often called its muscular or fibrous coat, is composed of many threads or fibres running in an obliquely transverse direction, and encircling the vessel. I say encircling, but remember that no one of these fibres is a complete ring; each, on the contrary, forms but a portion of the circle, being of a semilunar or crescentic shape, though by their union they effect the tubular form of the vessel.

Each fibre, then, of this middle coat is formed thus, (, not a circle, but only a portion of one; and a number of these conjoined complete the annular form.

These fibres of the middle arterial coat are of a peculiar nature, and though often called muscular, they have no resemblance farther than their mere fibrous structure, to the fleshy and true muscular fibre, and to this hour it remains a disputed point whether they possess

muscularity or not. Their colour is whitish yellow, and the coat is so thick, that when an artery is cut across it stands with open mouth; the fibres are all transverse, none being longitudinal.

The outer coat, then, of an artery gives it strength and toughness, and the middle elasticity, by which it preserves its open state, and thus offers little resistance to the onward course of the blood. Indeed, after all that has been said respecting the muscularity of this middle coat, it is not improbable that its peculiar substance and structure may have been given chiefly for this purpose, and that while the windpipe is provided with numerous rings of cartilage to render it always pervious to the air in breathing, so the tendency of the middle coat to preserve the open state of the arteries will essentially relieve the efforts of the heart to propel the blood through them.

I have farther to observe, that the middle coat may be separated into several layers; but there is no reason to believe that these are naturally distinct, like the coats of an onion, but that they depend on the facility with which the fibres allow themselves to be detached from each other, and hence, the more pains that are

taken, the more layers may be formed, but they are all artificial.

The inner coat of the arteries is extremely thin, dense, and smooth. It has a polished appearance, and offers no asperities to obstruct the passage of the blood along its surface. It is destitute of vessels; but by this you must understand that its vessels are so small as not to admit the finest injection, for there is no part, perhaps, of the animal frame unconnected with the circulating system, unless, as before said, we may except the enamel of the teeth.

The three coats of the arteries now mentioned, are connected to each other by fine cellular substance which some writers have dignified with the name of distinct coats, but to this they are not entitled, and you will recognise this connexion as another example of the office of the cellular membrane in being the medium by which the various parts of the animal structure are united together, no matter how much these may vary in their nature and functions.

VEINS.-While the arteries are the tubes which carry the blood to all parts from the left or systemic heart, the veins form the channels by which it is again returned to the right or

pulmonic heart, and in one circumstance they differ most materially from the arteries, viz. they have no middle fibrous coat. In consequence of this, when you cut a vein across, its sides collapse, come in contact, and do not shew an open mouth like an artery.

Still, however, though a vein has no resiliency in itself sufficient to retain its caliber patent, yet it may be so supported by its connexion to neighbouring parts as to stand open-mouthed when divided. This is the case in some of the veins of the liver, for instance, and, on making a section of this viscus, you will see many veins standing as patent or open-mouthed as arteries, but this arises from their connexion to the parts immediately around them.

The

The veins, then, are considered as having only two coats, an outer of condensed cellular substance, and an inner, very fine, dense, and smooth; it is supposed, however, that in the large veins near the heart, some longitudinal fibres, thought to be muscular, exist. veins are more capacious than the arteries, they have a greater number of large trunks, and are more irregular. Their coats are very thin, and of a watery bluish colour, owing to which, and the colour of their contained blood

VALVES OF VEINS.

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shining through them, they appear, when lying immediately under the skin in a living person, of a fine azure hue.

Suppose, now, that you wished to inject the arteries of the arm:-by inserting the pipe of the syringe into the trunk of the vessel in the arm-pit, that is, into the axillary artery, you will inject all its branches to the finger-ends. But, if instead of the artery you attempt to inject the veins of the arm from the axillary vein, which lies equally in the arm-pit, your injection will not, perhaps, run a couple of inches. You never could fill the veins either of the arm or leg from their main trunk, but the arteries of any part of the body you can best inject from their trunks.

Now, how is this to be accounted for? Why will the injection run freely from the axillary artery to the finger ends, and not equally fill the veins from their axillary trunk? The reason is this: there are no valves in the arteries, as has already been stated, except those at the origin of the pulmonary artery and of the aorta; but in the veins of the extremities there are numerous valves, and these are so constructed as completely to prevent regurgitation of their contents; though they offer no obL