1.—Cross section of a large artery showing the division into the three coats; intima, media, adventitia. The intima is a thin line composed of endothelial cells. The wavy elastic lamina is well seen. The thick middle coat is composed of muscle fibers and fibroelastic tissue. The loose tissue on the outer (lower portion of cut) side of the media is the adventitia. (Microphotograph, highly magnified.)

The muscular layer varies from single scattered cells, in the arterioles, to bands of fibers making up the body of the vessel in the medium-sized arteries and veins.

There is elastic tissue in all but the smallest arteries, and it is also found in some veins. It varies in amount from a loose network to dense membranes. In the intima of the larger arteries the elastic tissue occurs as sheets, which under the microscope appear perforated and pitted, the so-called fenestrated membrane of Henle.

The nutrient vessels of the arteries and veins, the vasa vasorum, are present in all the vessels except those less than one millimeter in diameter. The vasa vasorum course in the external coat and send capillaries into the media, supplying the outer portion of the coat and the externa with nutritive material. The nutrition of the intima and inner portion of the media is obtained from the blood circulating through the vessel. Lymphatics and nerves are also present in the middle and outer layers of the vessels.

Arteries

The structure of the arteries varies notably, depending upon the size of the vessel. A cross section of the thoracic aorta reveals a dense network of elastic fibers, occupying practically all of the space between the single layer of endothelial cells and the loose elastic and connective tissue network of the outer layer. Smooth muscle fibers are seen in the middle coat, but, in comparison with the mass of elastic tissue, they appear to have only a limited function.

In a cross section of the radial artery one sees a wavy outline of intima, caused by the endothelium following the corrugations of the elastica. The endothelium is seen as a delicate line, in which a few nuclei are visible. The media is comparatively thick, and is composed of muscle cells, arranged in flat bundles, and plates of elastic tissue. Between the media and the externa the elastic tissue is somewhat condensed to form the external elastic membrane. The adventitia varies much in thickness, being better developed in the medium-sized than in the large arteries. It is composed of fibrous tissue mixed with elastic fibers.

"Followed toward the capillaries, the coats of the artery gradually diminish in thickness, the endothelium resting directly upon the internal elastic membrane so long as the latter persists, and afterward on the rapidly attenuating media. The elastica becomes progressively reduced until it entirely disappears from the middle coat, which then becomes a purely muscular tunic, and, before the capillary is reached, is reduced to a single layer of muscle cells. In the precapillary arterioles the muscle no longer forms a continuous layer, but is represented by groups of fiber cells that partially wrap around the vessel, and at last are replaced by isolated elements. After the disappearance of the muscle cells the blood vessel has become a true capillary. The adventitia shares in the general reduction, and gradually diminishes in thickness until, in the smallest arteries, it consists of only a few fibroelastic strands outside the muscle cells." (Piersol's Anatomy.)

The large arteries differ from those of medium size mainly in the fact that there is no sharp line of demarcation between the intima and the media. There is also much more elastic tissue distributed in firm bundles throughout the media, and there are fewer muscle fibers, giving a more compact appearance to the artery as seen in cross section. The predominance of elastic tissue permits of great distention by the blood forced into the artery at every heartbeat, the caliber of the tube being less markedly under the control of the vasomotor nerves than is the case in the small arteries, where the muscle tissue is relatively more developed. The adventitia of the large arteries is strong and firm, and is made up of interlacing fibroelastic tissue, of which some of the bundles are arranged longitudinally.

Veins

The walls of the veins are thinner than those of the arteries; they contain much less elastic and muscular tissue, and are, therefore, more flaccid and less contractile. Many veins, particularly those of the extremities, are provided with cup-like valves opening toward the heart. These valves, when closed, prevent the return of the blood to the periphery and distribute the static pressure of the blood column. The bulgings caused by the valves may be seen in the superficial veins of the arm and leg. There are no valves in the veins of the neck, where there is no necessity for such a protective mechanism, gravity sufficing to drain the venous blood from the cranial cavity.

Capillaries

These are endothelial tubes in the substance of the organs, the tissue of the organ giving them the necessary support. They are the final subdivisions of the blood vessels, and the vast capillary area offers the greatest amount of resistance to the blood flow, thus serving to slow the blood stream and allowing time for nutritive substances or waste products to pass from and to the blood. Usually the capillaries are arranged in the form of a network, the channels in any one tissue being of nearly uniform size, and the closeness of the mesh depending upon the organ.

As far back as 1865, Stricker observed contraction of the capillaries. This observation was apparently forgotten until revived again by Krogh recently. The latter finds that the capillaries are formed of cells which are arranged in strands encircling the vessel. The capillaries are rarely longer than 1 mm., and, according to Krogh, are capable of enormous dilatation.

The rate of flow through any capillary area is very inconstant, and the usual explanation has been that the capillaries were endothelial tubes the blood flow of which was dependent upon the contraction or dilatation of the terminal arterioles. The actual fact that in an observed capillary area some capillaries are empty renders the above explanation untenable. The color of a tissue depends upon the state of filling of the capillaries with blood.

It would seem that all the evidence now leads us to believe that the capillaries themselves are contractile and it is even possible that they may be under vasomotor control. If the anatomic structure as stated above, is correct, it would take but a slight contraction of the encircling cell to shut off completely the capillary. When the enormous capillary bed is considered, it is not inconceivable that circulating poisons may act on large areas and produce a true capillary resistance to the onflow of blood which might express itself, if long continued, in actual hypertrophy of the heart.

CHAPTER II.

PATHOLOGY

The whole subject of the pathology of arteriosclerosis has been much enriched by the study of the experimental lesions produced by various drugs and microorganisms upon the aortas of rabbits. Simple atheroma must not be confused with the lesions of arteriosclerosis. The small whitish or yellowish plaques so frequently seen on the aorta and its main branches, may occur at any age, and have seemingly no great significance. Such plaques may grow to the size of a dime or larger, and even become eroded. They represent fatty degeneration of the intima which, at times, has no demonstrable cause; at times follows in the course of various diseases,