28 



THE REASON WHY: 



&quot; All men think all men mortal but themselves ; 

 Themselves, when some alarming shock of fate 

 Strikes through their wounded hearts the sudden dread.&quot; YOUNG. 



place by this circulating spring of energy and nourishment. No 

 vital action is maintained without blood, and should it cea&e to 

 flow through the brain, all the sense would be speedily shut up, 

 and every function speedily superseded. 



74. In the accompanying 1 engraving the circulation of the blood as performed by 

 a single heart is depicted : V represents the ventricle, or strong muscular bag of th 

 heart, which when filled with blood contracts upon it, just as any other muscle 

 does, and so forces out the contents through the pipe which arises from it, called 

 the aorta, just as the contents of an india-rubber bag are 

 squeezed out through a pipe fixed in its neck. The only 



Ol 1 difference is, that whereas an external force squeezes the 

 bag, the heart, beir.g muscular, has a power of contraction 

 of its own, and, as it were, squeezes itself ; and then, just 

 like the india-rubber bag which regains its shape when the 

 pressure is removed, so the heart, when it has squeezed 

 out all the blood, dilates itself again, and is ready to con 

 tract anew. The blood having been poured into the great 

 artery, goes through branches up to the head, and down to 

 the lower part of the body, where its minute or capillary ter 

 minations are seen to end in veins. Those from the lower 

 part f *Ha body form an inferior great vein ; those from 

 the upper, * superior : and the two veins terminate 

 separately into a bag A, called the auricle. The auricle is 

 not nearly so sti ong as the ventricle, because it has nothing to 

 with f ()rc i n g the blood over the body; it is in tended merely 

 as a receptacle for the venous blood, till the ventricle be ready to 

 receive it. The auricle is constantly full of blood, which flows 

 to it through the veins in an equable stream, so that when 

 ever the emptied ventricle dilates, the blood from the 

 auricle rushes in, and distends it for a renewed contraction. 

 The arteries are a set of tubes both dilatable and elastic. Hence at the moment 

 when the ventricle contracts, the blood which is forced into them distends them, 

 increasing their diameter, and producing the feeling communicated to the fingers 

 placed over them, which is called the pulse. The number of the pulse is therefore 

 the number of contractions which the heart is making in a minute ; and at the 

 moment when the ventricle dilates, the artery, having the distending force taken 

 off, contracts on its contents. It would now drive part of the blood back again 

 into the ventricle were it not for a valve placed in the artery at its origin, which 

 shuts down the moment the pressure comes on it backwards, so that the force of 

 the elasticity of the artery is expended in propelling the blood forward, not in an 

 equable stream but in successive waves. Again ; when the ventricle contracts tc 

 throw its blood into the aorta, it would throw back an equal portion into the 

 auricle, were not a valve placed there also, which shuts the moment the ventricle 

 on tracts. 



