Part 22 (2/2)

[Illustration: FIG 174]

THE FORCE-PUMP

[Illustration: FIG 175 Force-pump; suction stroke]

[Illustration: FIG 176 Force-pu water to levels above that of the pues 175, 176 The piston now is solid, and the upper valve is situated in the delivery pipe During an upstroke this closes, and the other opens; the reverse happening during a downstroke An air-chaenerally fitted to the delivery pipe ater is to be lifted to great heights or under high pressure At each delivery stroke the air in the chaiven to the water in the pipe by the water co the next suction stroke forces the water gradually up the pipe The air-chaine

A _double-action_ force-pu an upward stroke Both sides of the piston are here utilized, and the piston rod works through a water-tight stuffing-box The action of the puram

[Illustration: FIG 177]

THE MOST MARVELLOUS PUMP

known is the _heart_ We give in Fig 178 a diagrammatic sketch of the syste the heart, the arteries, and the veins, big and little The body is supposed to be facing the reader, so that the left lung, etc, is to his right

[Illustration: FIG 178--A diagrammatic representation of the circulatory system of the blood]

The heart, which forces the blood through the body, is a large muscle (of about the size of the clenched fist) with four cavities These are respectively known as the right and left _auricles_, and the right and left _ventricles_ They are arranged in two pairs, the auricle uppermost, separated by a fleshy partition Between each auricle and its ventricle is a valve, which consists of strong es turned doards The left-side valve is the _ht auricle and ventricle the _tricuspid_ valve The edges of the valves fall together when the heart contracts, and prevent the passage of blood Each ventricle has a second valve through which it ejects the blood (That of the right ventricle has been shown double for the sake of convenience)

The action of the heart is this:--The auricles and ventricles expand; blood rushes into the auricles fro them, and distends them and the ventricles; the auricles contract and fill the ventricles below quite full (there are no valves above the auricles, but the force of contraction is not sufficient to return the blood to the veins); the ventricles contract; theto the arteries open; blood is forced out of the ventricles

THE BLOOD CHANNELS

are of two kinds--(1) The _arteries_, which lead the blood into the circulatory system; (2) the _veins_, which lead the blood back to the heart The arteries divide up into branches, and these again divide into smaller and smaller arteries The smallest, termed _capillaries_ (Latin, _capillus_, a hair), are e diameter of 1/3000th of an inch These permeate every part of the body The capillary arteries lead into the ser veins, until e h these the blood flows to the heart

There are three main points of difference between arteries and veins In the first place, the larger arteries have thick elastic walls, and maintain their shape even when empty This elasticity performs the function of the air-cha blood into the arteries, the walls of the latter expand, and their contraction pushes the blood steadily forithout shock The capillaries have very thin walls, so that fluids pass through the out waste matter The veins are all thin-walled, and collapse when empty Secondly, most veins are furnished with valves, which prevent blood flowing the wrong way

These are similar in principle to those of the heart Arteries have no valves Thirdly, arteries are generally deeply set, while many of the veins run near the surface of the body Those on the front of the arm are specially visible Place your thu towards the wrist, and you will notice that the veins distend owing to the closing of the valves just mentioned

Arterial blood is _red_, and coulps, on account of the contraction of the elastic walls If you cut a vein, _blue_ blood issues in a steady strea the passage of the blood through the capillaries, and the absorption of carbon dioxide froreat purifiers of the blood As it circulates through theives up the carbon dioxide which it has absorbed, and receives pure oxygen in exchange If the air of a rooen For this reason it is advisable for us to keep the s of our rooue is caused by the accumulation of carbon dioxide and other impurities in the blood When we run, the heart pus faster than they can purify it, and eventually our muscles become poisoned to such an extent that we have to stop from sheer exhaustion

THE COURSE OF THE BLOOD

It takes rather less than a h the whole system and back to the heart

We may briefly summarize the course of the circulation of the blood thus:--It is expelled from the left ventricle into the _aorta_ and the main arteries, whence it passes into the smaller arteries, and thence into the capillaries of the brain, stoen to the body, and takes in ih theht ventricle; is expelled into the _puls, and is purified It returns to the left auricle through the _pulmonary veins_; enters the left auricle, passes to left ventricle, and so on

A healthy heart beats from 120 times per ed person The nored adult is fronifies the failure of the heart valves to close properly Blood passes back when the heart contracts, and the circulation is h a stethoscope the doctor is able to tell whether the valves are in good order A hissing sound during the beat indicates a leakage past the valves; a thump, or ”clack,” that they shut completely

THE HYDRAULIC PRESS