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1.Have you ever noticed that when you were ill the first thing the doctor did when he came to see you was to take up your hand and place histwo fingers firmly on your wrist？Perhaps you know why he did so：it was in order that he might feel your pulse.If you place the points of your two first fingers on the wrist of your other hand，a littleway above the thumb，and press firmly，you will feel a throbbing movement under the skin.
2.Do this again after you have been running，and you will find the throbbing movement much more rapid，andalso stronger，as if the moving part were able to resis.aharder pressure.Place your hand next on the left side of your chest，and you will feel throbbing movement there which keeps time exactly with that in your wrist.
3.So the pulse in your wrist tells the doctor the rate at which your heart is beating，and it also tells him thestrength with which the heart is doing its work.In many kinds of disease the heart beats too rapidly，and at the same time the pulse feels feeble，and wanting in firmness.
4.We must now try to understand how the heart does its work，and what the use of that work is to the body.The work of the heart is to send the blood to every part of the body.The blood flows away from the heart in strong tubes called arteries.These arteries divide up into branches，which again divide andsubdivideinto smaller tubes.At last they are brokenup into a network of tubes or blood-vessels called capillaries，which are finer than hairs.The capillaries join again to form small tubes called veins，and these small veins unite like the tributaries of a river to form the large veins which carry the blood back to the heart.
5.All movements in the body are caused by con-tractionof muscle，and the movement of the bloodis due to this cause also.The heart is a hollow bag of strong muscle.Its movements will be most easily understood by thinking of it first as having a single chamber with two doors-one for the blood to enter from the veins，and one for it to leave by the arteries.When the heart is at rest，the blood pours into it fromthe veins.Then it suddenly contracts.One would think that the blood would be squeezed out through bothopenings，but there is a valveor trap-door whichprevents it from getting back into the veins，so it is forced through the other door into the arteries.
6.Then the muscles of the heart become slack once more.Blood pours into it from the veins ；but a valve at the entrance to the arteries prevents any from coming back that way，so it is filled from the veins alone.The next contraction forces this blood along the arteries in the same way as before，and thus the current is kept up.Every contraction or beat of the heart sends a fresh wave along the arteries，and it is this wave which we call the pulse.
7.But the heart is not by any means so simple in its form as we have so far thought of it.In the first place，the blood does not enter the large chamber or ventricle direct from the veins；it gathers in an entrance-chamber or lobby above，called the auricle.While theventricle is dischargingone portion of blood，anotherportion is getting ready for it in this entrance-chamber.There is a trap-door between the two chambers which opens downwards only，so that blood cannot get back from the ventricle to the auricle again.
8.We have spoken of it so far as a single organ，butthe heart is really a double organ，or as we might say，consists of two hearts joined together-a right heart and a left.It has a right and a left division，each with its own auricle and ventricle，and there is no directcommunicationbetween these two divisions.Thereason for this is that we have two circulations in our body.By the first the blood is sent all through the body，to nourish and warm it；by the second it is sent through the lungs，to be purified and supplied with oxygen to prepare it for its next journey，as we shall see in another lesson.