The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

into the surrounding lymph-spaces. This is how lymph originates and how the supply of lymph is sustained.

For the return of the lymph to the blood there is a system of vessels, very thin-walled and delicate, which actually open into the lymph-spaces, so that the fluid can escape along them. These vessels are called lymphatis. ‘They unite with each other, like veins, to form larger vessels, and, like veins, they guide their contained fluid, which is simply filtered blood-plasma, slowly upward

The Organs of Respiration. The left lung is dissected to show the bronchi.

Fig. 19.

towards the heart. They are indeed shadowy plasma veins.

The flow of lymph in the lymphatics is slower even than that of blood in the veins, and, as in the veins, it is assisted by valves, by bodily movements, and probably by rhythmical contraction of the vessels themselves. The course of the lymphatic vessels is not so direct as that of the veins. ‘They unite and separate again to form networks and their course is interrupted by passages through curious spongy labyrinthine filters, the “lymph-glands,” but ultimately the lymph is led to a great main lymphatic in

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THE SCIENCE OF LIFE

CHAPTER 2

the chest, the thoracic duct, and this trunk runs up the back of the chest and opens into one of the great veins near the heart. Thus the lymph is returned again to the blood-stream from which it originated.

Besides merely collecting the blood which oozes out of the capillaries and returning it to the circulation, we may note here that the lymphatic system has some interesting special functions connected with the absorption of food, to which we shall return later.

§ 5 Breathing

We have already made it plain how and why man must breathe, and little need be said here about the machinery of breathing. It would take about three minutes to suffocate Mr. Everyman. We all know that the lungs are two spongy organs, lying to the sides of and behind the heart and that together with that organ they fill the chest. To the cat lover their appearance will be familiar, and they will be better known as “lights.’? Lights is the old word for lungs; in the bills of mortality of the eighteenth century, pulmonary consumption figures as “rising of the lights.” They communicate through the windpipe and the mouth or the nose with the outer air.

The windpipe (trachea) is a strong tube, about two-thirds of an inch across, which opens into the mouth at the back of the throat. The walls of the trachea are strengthened and made rigid by rings of gristle, so that whatever movements the neck may make this tube stays widely open. The upper part, just below the epiglottis, is modified to form the larynx, Adam’s apple, or voicebox, but this structure plays no part in the respiratory function. It is a wind instrument inserted at this point to take advantage of the air-rush.

The trachea runs down the front of the neck into the chest, where it divides into two branches (broncht)—one to each lung. The bronchi, when they reach the lungs, divide again, and their branches divide ; ultimately, by continual division, they lead into minute air-channels, less than a hundredth of an inch across, which permeate the substance of the lungs. Out of these