CHAPTER IV.

The blood-The heart-The blood-vessels-The general circulation— Pulmonary circulation-Collateral circulation-The pulse and its variations-Vital temperatures-Local temperature.

IN view of the definition of health which we have quoted-the perfect circulation of pure blood in a sound organism-it becomes desirable for us to know something of the nature of pure blood, and of the means by

FIG. 2.-Red and white corpuscles of the blood. Magnified.

which its circulation is carried on. It is the most abundant, as well as the most important fluid of the body,

pervading nearly every part of the system; upon its presence and its unceasing motion, life as well as health depends. It appears to the naked eye as a simple red fluid, but examined under the microscope, it is seen to be made up of a multitude of little solid bodies, floating in a clear colorless liquid. They are called corpuscles, literally little bodies, and the liquid in which they float is known as plasma. The plasma is made up of serum and fibrin. The corpuscles are mostly of a yellowishred hue, and it is from their vast numbers that the blood derives its red appearance. There are some white ones. They are larger than the red, and of a different shape, but comparatively few in number.

The blood while it circulates through the body is, though somewhat glutinous, perfectly fluid, but, upon removal from its natural surroundings, it exhibits a wellknown tendency to coagulate or solidify. The fibrin of the plasma separates itself from the serum, and entangles the floating corpuscles into a mass. This peculiarity affords protection against undue loss of blood, for dangerous hæmorrhage would follow even a slight cut, did not the clots thus formed effectually close the injured blood-vessels and prevent further escape of the vital fluid. Occasionally this coagulation of the fibrin takes place while the blood is still in motion through the vessels, obstructing the circulation very seriously. This is called thrombosis. A clot so formed, or any other solid body, arrested in the arteries or capillaries, constitutes an embolus.

The office of the blood is to convey nutrition to all parts of the body, and to remove its waste material. The way in which it circulates was discovered, early in the seventeenth century, by William Harvey. The process is carried on by means of the heart, and blood-ves

sels of three distinct kinds: arteries, which carry the blood away from the heart; veins, which bring it back to the heart; and capillaries, connecting the two.

The heart is a pyramidal organ, situated nearly in the center of the chest. The apex, pointing downward, forward, and to the left, can be felt between the fifth and sixth ribs. The base is on a level with the upper border of the third rib. The base is fixed, but the apex

is freely movable. The heart is composed of muscular fiber. It is enveloped in a fibro-serous membrane, called the pericardium, which secretes a lubricating fluid enabling its movements to be accomplished without loss of power by friction. It is hollow, and partitioned into four

cavities or chambers of nearly equal capacity, two at the base called auricles, and two below termed ventricles.

FIG. 4.-Cavities of the heart: od, right auricle; vd, right ventricle; og, left auricle; vg, left ventricle. The arrows indicate the course of the blood.

There is no opening between the ventricles. A valve between the two auricles closes at birth, and gradually disappears, after which there is no longer any connection between the two sides of the heart. The left side always contains pure, the right side impure, blood. If the valve between the two auricles fails to close when independent circulation is established, the impure blood mixes with the pure, and we get what is called a "blue baby," a condition soon fatal. But between each auricle and its corresponding ventricle there is an orifice, guarded by a valve, which permits the passage of fluid in but one direction-downward. The valve between

the right auricle and the right ventricle, is called the tricuspid valve; that at the left auriculo-ventricular aperture, the bicuspid, or, more commonly, the mitral valve. Each ventricle has also another opening, provided with a set of "semi-lunar" valves, connecting it with a large artery, the aorta on the left, and the pulmonary on the right. The auricles also have other openings through which the blood flows into them from the great veins, but they are not supplied with valves. As the auricles become filled, they contract, and the blood, following the line of least resistance, is forced into the ventricles. They in turn similarly contract, forcing it on into the arteries, regurgitation being in each case prevented by the intervening valves. The sounds heard upon auscultation are produced by the closing of these valves. Then follows a pause, after which the contractions are repeated in the same order, and followed again by the same period of repose, during which the cavities undergo gradual dilatation. The pause occupies about as much time as the two contractions, the entire action less than one second. The state of contraction of the ventricle, or auricle, is called its systole, that of relaxation its diastole. Both sides of the heart act simultaneously.

Let us follow on its course the blood which is expelled by the left ventricle. The semi-lunar valves open to allow it to pass into the aorta, the main trunk of the arteries. This ascends from the upper part of the left ventricle for a short distance, then forms an arch backward over the root of the left lung, and passes down into the abdomen, where it is divided into two great branches. From every part of its length, it sends out similar branches. They all again divide and subdivide into numberless ramifications, extending to all parts of the body, and gradually diminishing in size as they be