CHAPTER VI.

THE VASCULAR SYSTEM.

THE blood-vessels compose a system of closed tubes, of mesoblastic origin, which are divided structurally into arteries, veins, and capillaries.

ARTERIES.

An artery has three coats, called respectively outer, middle, and inner. The inner coat, serous coat, or tunica intima, consists of a single layer of thin, flat, elongated endothelial cells, united by a delicate cement substance, which rests upon a subendothelial layer of fibrous tissue. Surrounding this layer of fibrous tissue is a wavy band of yellow elastic tissue. In the larger arteries this elastic tissue forms a thick sheet, in which more or less irregular openings occur. It is then called the fenestrated membrane of Henle.

The middle coat, muscular coat, or tunica media, surrounds this fenestrated membrane of Henle, and consists principally of smooth muscle, most of which runs circularly around the artery. Some few fibers of fibrous and yellow elastic tissue are also found interspersed among the muscle-cells, especially in the larger arteries.

The outer coat, fibrous coat, or tunica adventitia,

consists of fibrous tissue in rather dense bundles, with occasional fibers of yellow elastic tissue.

The inner coat presents a smooth surface, which offers scarcely any resistance to the blood current.

The muscular coat is under control of the vasomotor nerve system, and, by contracting or dilating, changes the size of the lumen, thereby regulating the amount of blood passing through a given vessel.

The outer coat gives strength to the artery, and serves to sustain it in its relations to the adjacent tis

Fig. 18.-Blood-vessel: A, Tunica intima; B, tunica media; C, tunica adventitia; a, endothelial lining; b, blood-corpuscles; c, fenestrated membrane of Henle.

VEINS.

The veins differ from the arteries only in the relatively small size of their muscular coat and the large amount of fibrous coat. Their walls are thinner in proportion to the lumen, and when cut they tend to collapse.

THE HEART.

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CAPILLARIES.

The larger arteries branch repeatedly until the finest arterioles are reached, when they lose their outer two coats and continue as thin tubes consisting of only a single layer of endothelial cells, the continuation of the endothelial lining of the larger vessels. Occasionally, the endothelial cells do not fit together exactly, and small spaces occur, which are filled with cement substance. These are called stigmata, and are probably the first places through which the white blood-corpuscles pass in inflammations.

The larger vessels are supplied with nutrition by smaller vessels, which ramify within their coats. These have received the name of vasa vasorum.

Lymphatics also occur in the outer coat of the larger vessels, and some of the smaller blood-vessels are completely surrounded by narrow lymphatic spacesthe perivascular lymphatics.

THE HEART.

The heart is lined by the endocardium, which is similar in structure and continuous with the tunica intima of the blood-vessels.

Under this is the myocardium, or heart muscle, which has already been considered.

The outer surface of the heart is covered by the pericardium, a serous membrane containing a membrane of fibrous tissue, on the free surface of which are flat, irregular endothelial cells.

The valves of the heart are broad, flat reduplications of the endocardium, between the two layers of which is a plate of very tough, dense fibrous tissue.

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cart contains many lymphatics, some of which ween the muscle-tibers; others are found around onary arteries,

modulated and nonmedullated nerve-fibers occur in abundance, the former being chiefly branches of the pneumoga arie; the latter, from the sympathetic sys

tem.

Numerous ganglia are also found in the course of the

THE BLOOD.

The blood may be considered as a mesoblastic tissue von ading of cells with a fluid intercellular substance.

The cellular elements of the blood are called corpuscles; the thuid portion, the plasma, or liquor Manguinis.

Pho principal corpuscles are the colored, the colorless, und the 400 AGIOVIN Other forms that occur are the dea vợ phaing the shadow corousvics, the microcytes, and The colored corpuscles, red erythrocytes, are round, biconcave When viewed ingly under the microscope, they leave a vollowish green color, the characteristic red color of blood bong produced only by the superposition of great number of cells. Their number varies omewhat in diferent individuals, and is slightly greater

mile dem in the female. A cubic millimeter (1⁄4, of in melo of blood in the male will contain, on an 3,000,000 colored blood cells, whereas the

made will average about 1,500,000.

by many de tod cells are singularly regular in size, of minch microns) in breadth and o inch (2.08 microns) in thickness at Paddockness of the center depends upon avay, and varies much in different

cells. In stained preparations the center appears clear, and is called the "delle."

Structurally, the colored cells are surrounded by a clear, extremely delicate limiting membrane (virtually a cell-wall), inclosing the body of the cell, the hemoglobin, which is the oxygen-carrying element. In the adult no nuclei are found in the colored cells, although in the embryo and a few days after birth some nucleated red cells occur. When allowed to remain quietly outside the blood-vessels, they tend to arrange themselves into stacks, or rouleaux, resembling rolls of coin.

Action of Reagents.

Upon the addition of water to the blood the red cells swell up until they become spheric in outline, and lose their hemoglobin, which is dissolved out.

When acted upon by dilute saline solutions, minute spines develop around the surface of the cell, giving it somewhat the appearance of a horse-chestnut bur. Corpuscles in this condition are said to be crenated. If the strength of the solution be increased, the cells will eventually shrivel up into an unrecognizable mass. Almost all acids, especially acetic acid, decolorize the cells and cause them to become spheroid in outline.

Tannic acid in a 1% solution causes the hemoglobin to escape from the cell and to coagulate in minute beads at the edge of the limiting membrane. If the solution used is too strong, the hemoglobin will be coagulated within the cell.

Colored Corpuscles in Other Animals.

The red blood-cells of all mammals (with the exception of the camel tribe) differ only in size. The members of the camel tribe, birds, and cold-blooded animals have oval nucleated colored cells.