3. Internal, composed of a thin homogeneous membrane, covered with a layer of elongated endothelial cells.

The arteries possess both elasticity and contractility.

The property of elasticity allows the arteries already full to accommodate themselves to the incoming amount of blood, and to convert the intermittent acceleration of blood in the large vessels into a steady and continuous stream in the capillaries.

The contractility of the smaller vessels equalizes the current of blood, regulates the amount going to each part, and promotes the onward flow of blood.

Blood Pressure.-Under the influence of the ventricular systole, the recoil of the elastic walls of the arteries, and the resistance offered by the capillaries, the blood is constantly being subjected to a certain amount of pressure. If a large artery of an animal be divided, and a glass tube of the same caliber be inserted into its orifice, the blood will rise to a height of about nine feet; or if it be connected with a mercurial manometer, the mercury will rise to a height of six inches. This height will be a measure of the pressure in the vessel. The absolute quantity of mercury sustained by an artery can be arrived at by multiplying the height of the column by the area of a transverse section of that artery.

The pressure of the blood is greatest in the large arteries, but gradually decreases toward the capillaries.

The blood pressure is increased or diminished by influences acting upon the heart or upon the peripheral resistance of the capillaries, viz. :—

If, while the force of the heart remains the same, the number of pulsations per minute increases, thus increasing the volume of blood in the arteries, the pressure rises. If the rate remains the same, but the force increases, the pressure again rises. Causes that increase the peripheral resistance by contracting the arterioles, e.g., vasomotor nerves, cold, etc., produce an increase of the pressure.

On the other hand, influences which diminish either the volume of the blood, or the number of pulsations, or the force of the heart, or the peripheral residence, lower the pressure.

The Pulse is the sudden distention of the artery in a transverse and longitudinal direction, due to the injection of a volume of blood into the arteries at the time of the ventricular systole. As the vessels are already full of blood, they must expand in order to accommodate themselves to the incoming volume of blood. The blood pressure is thus increased, and the pressure originating at the ventricle excites a pulse wave, which passes

from the heart toward the capillaries at the rate of about twenty-nine feet per second. It is this wave that is appreciated by the finger.

The Velocity with which the blood flows in the arteries diminishes from the heart to the capillaries, owing to an increase of the united sectional area of the vessels, and increases in rapidity from the capillaries toward the heart. It moves most rapidly in the large vessels, and especially under the influence of the ventricular systole. From experiments on animals, it has been estimated to move in the carotid of man at the rate of sixteen inches per second, and in the large veins at the rate of four inches per second.

The Caliber of the Blood-vessels is regulated by the vasomotor nerves, which have their origin in the gray matter of the medulla oblongata. They issue from the spinal cord through the anterior roots of spinal nerves, pass through the sympathetic ganglia, and ultimately are distributed to the coats of the blood-vessels. They exert at different times a constricting and dilating action upon the vessels, thus keeping up the arterial tonus.

Capillaries. The capillaries constitute a network of vessels of microscopic size, which distribute the blood to the inmost recesses of the tissues, inosculating with the arteries on the one hand and the veins on the other; they branch and communicate in every possible direction.

The diameter of a capillary vessel varies from the to the 3000 of an inch; their walls consist of a delicate homogeneous membrane, the 20 of an inch in thickness, lined by flattened, elongated, endothelial cells, between which, here and there, are observed stomata.

It is through the agency of the capillary vessels that the phenomena of nutrition and secretion takes place, for here the blood flows in an equable and continuous current, and is brought into intimate relationship with the tissues, two of the essential conditions for proper nutrition.

The rate of movement in the capillary vessels is estimated at one inch in thirty seconds.

In the capillary current the red corpuscles may be seen hurrying down the center of the stream, while the white corpuscles in the still layer adhere to the walls of the vessel, and at times can be seen to pass through the walls of the vessel by ameboid movements.

The Passage of the Blood through the capillaries is mainly due to the force of the ventricular systole and the elasticity of the arteries; but it is probably also aided by a power resident in the capillaries themselves, the result of a vital relation between the blood and the tissues.

The Veins are the vessels which return the blood to the heart; they