has ruptured them. 2. The middle coat is of a buff colour and is made of two elements-the elastic, abounding in the large arteries, and the muscular in the smaller. The yellow elastic fibres are arranged in an outer circular layer, which constitutes of the whole thickness of the vessel. This is mixed up with branching or penniform fibres of Todd and Bowman, whose section is here

copied, and organic muscular fibres. Still more elastic tissue is arranged in longitudinal fibres, that just under the lining membrane, of which it is the basement layer, being the "fenestrated coat" of Henle. 3. The inner coat consists of long epithelial cells with distinct nuclei, and is in immediate contact with the blood, which must nourish it and receive the epithelial cells as they are exuviated. Morbid alterations occur in and under this membrane.

Arteries divide into gradually smaller branches at a right angle near the heart, and obliquely far from it, where the force is diminished; and as the area of the branches is mostly greater than the trunk they spring from, the whole arterial system assumes the form of a cone. This enlargement is best marked in small arteries, where the area almost doubles; whereas in the com

anastomosis of arteries is accomplished by the direct joining of two trunks into one, as vertebrals; by their being connected by intermediate vessels, as at the circle of Willis; by dividing so as to form arches, as in the mesentery. The figure on opposite page shows this method of anastomosis, which prevails in all the digestive viscera. Still more frequently, arteries communicate by small branches being given off, which join those from the vessel lower down, or from the opposite side of the limb. It is such inosculations which are so useful in preserving the collateral circulation if by disease or ligature an artery becomes obstructed.

The Elasticity of the arteries Hunter proved by the following experiment: "A circular section of the aorta ascendens, when slit up and opened into a plane, measured 5 inches; on being stretched, it lengthened to 10 inches; the stretching power being removed, it contracted again to 6 inches, which we must suppose to be the middle state of the vessel." This property diminishes with the size of the vessel; the muscularity increases. Suppose an artery full of blood, the systole of the left ventricle must force more blood in, and this must be accommodated by the forcing of the blood onwards into the capillaries, or by the increase of diameter of the artery, which, as may be calculated by placing an elastic metal ring round the vessel, is equal to of its previous calibre. During the succeeding diastole the elasticity of the artery re-acts on the blood, and the aortic valves closing, the stream must be moved onwards. Insufficiency of supply and gangrene of a limb will follow if elasticity is impaired, as by the calcification of the artery. By elasticity also the intermitting pulsations of the heart are converted into a constant stream, as achieved in the common fire-engine by a like elasticity of the hose. The blood, however, from a divided small artery jets "per saltum," showing that a perfectly uniform current is not obtained. If a canula be inserted into the

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ventricle of a living animal, the blood will also jet out, but if it be inserted into the aorta, which is so elastic, a uniform flow occurs. An equable and prolonged supply is also obtained by the artery breaking up into many minute branches, which again re-unite, as seen in the "rete mirabile" of Galen on carotid and vertebrals of animals which graze, preventing congestion; in the rete ophthalmicum of birds; in the arm of the sloth, which allows the animal to remain suspended by the limbs for hours; and in the leg of the swan, goose, &c., which permits prolonged standing. The same object is attained by great tortuosity, as in the seal, where the carotid is nearly 40 times longer than the space it traverses. During systole arteries become much more tortuous, and this condition becomes permanent in old people, as their arteries harden. The coats of the vessel are always thicker at its convexity.

The Irritability of the arteries was also shown by Hunter in the following and many other experiments : "The posterior tibial artery of a dog being laid bare, and its size attended to, it was observed to be so much contracted in a short time as almost to prevent the blood from passing through it, and when divided, the blood only oozed out from the orifice." Besides air, as in this case, electricity and mechanical stimulus will excite muscular contraction in the smaller arteries, and if applied at interrupted points will actually produce a monilar or beaded appearance. Müller denied these statements, grounding his opinion on the structure of very large arteries. The usual tests also show the presence of an albuminoid, which fibrous tissue has not. Muscularity of the arteries does not afford much propelling force, but rather regulates the due supply of blood to any part. It does not therefore exist in large arteries, as no cause requiring an increase or decrease of blood could be so general as to affect all the parts to which, for instance, the brachio-cephalic artery sends blood. It is owing to

the suspension of this tonic power that increase of blood is allowed in erectile or occasional organs. Contractility will remain for even 48 hours after death, as in the famous experiments of Hunter on the umbilical cord, and thus renders the injection of a subject more difficult at this period. Irritating fluids will pass with difficulty through vessels recently deprived of life, but bland ones will pass with rapidity and ease. The sympathetic nerves supply the irritability, as contractions can be produced by irritating them. Cold increases, heat decreases tonicity. Rhythmical pulsations have been observed in the arteries of the ears of rabbits, analogous to those which Wharton Jones discovered in the veins of the bat's wing, and, like them, not synchronous with the cardiac or respiratory movements.

III. The Capillaries were discovered, in 1661, by Malpighi, and till then there

was a gap in Harvey's theory. Their name does not express their fineness, as they measure but from Too to doo; the smaller are found in retina and brain, the larger in liver and lungs, which are here figured. They preserve the same width through the whole length from artery to vein. Wedemeyer denied their separate existence, stating the artery joined the vein merely by canals hollowed in the substance of the tissues they permeate. High microscopic power has, however, proved that they have distinct walls, but has failed to exhibit any structure in such coats, which are therefore similar to cell-wall and sarcolemma. A nucleus is sometimes seen in them. The question of the vascularity or non-vascularity of certain tissues is but one of degree, for even in the most vascular the capillaries do

Capillaries of Lung.