106 PHYSIOLOGY AND HEALTH. 



force sufficient to send them through the arteries to the 

 farthest extremity of the frame. The power of the heart is 

 not easily measured. Some have supposed that it could 

 exert a force equal to that which would be necessary to raise 

 several thousand pounds ; while others have estimated it to 

 be equal only to a few ounces. 



228. The veins are continually pouring their blood into 

 the right auricle, or upper chamber. As soon as this is full, 

 it contracts, and empties its contents into the right ventricle, 

 or chamber below. In the same manner, when the pulmo- 

 nary veins fill the left auricle, this presses the blood into the 

 chamber below ; and then this lower cavity contracts and 

 forces it into the arteries, and through them to the body. 



229. This movement of the blood is always forward. The 

 great vein pours its blood into the right auricle. When this 

 upper chamber is filled, it contracts upon its contents, arid 

 " the reflux of the blood into the veins is prevented by the 

 valves with which they are furnished ; " but at the same 

 time the valve between this cavity and the ventricle below is 

 opened, and the blood finds free passage through it. When 

 this lower chamber is filled and contracts, the last valve is 

 closed, and the fluid has no way of going back to the upper 

 chamber ; but then the valve between the ventricle and the 

 artery opens to allow the blood to enter this tube, and closes 

 again as soon as the artery is full ; so that the blood cannot 

 go back to the heart. In the same manner, the valves on 

 the left side of the heart open to allow the blood to pass 

 from the veins of the lungs to the left auricle, and from the 

 auricle to the lower chamber, and again from this chamber 

 to the arteries of the body ; but at each place they close 

 when the next cavity is filled, and prevent the return of any 

 fluid 



239. The arteries are capable of expansion . and con- 

 traction. When any thing is forced into them, their coats 

 stretch, and their capacity is enlarged; and when their con- 

 tents are removed, they contract again, and diminish theii 

 cavity. These have no valves, except that which stands 



