The Heart. 57 



ventricle of the horse as equal to a column of blood 5'35 

 feet in height : in the right ventricle as equal to one 1*04 

 feet in height. According to Colin's experiments, made 

 with a manometer in the form of a long tube, the left ven- 

 tricle is capable of sustaining a column of blood 656 feet 

 high, whilst the force in the right ventricle will support in 

 the pulmonary artery a column of blood 164 feet high. 

 This, calculated on the surface of the ventricle, is equal to 

 a force of 72'6 lbs. for the right ventricle, and 290-4 lbs. for 

 the left (Colin). The right ventricle does about one-fourth 

 less work than the left. Munk gives the daily work of the 

 heart of the horse as equal to one-thirty-sixth of a horse- 

 power, or nearly 15-3 lbs. lifted one foot high per second. 



The term Blood Pressure is frequently used in the physio- 

 logy of the circulatory system. It is one we shall have to 

 consider in detail when we come to speak of the blood- 

 vessels ; but we should clearly understand that the condi- 

 tion is due to the amount of blood pumped into the vessels 

 by the heart, and this amount will depend principally on 

 the rate at which that which is in front of it in the vessels 

 escapes into the veins. If the arterioles are contracted so 

 that the amount passing into the veins is reduced in 

 quantity, then a larger bulk of blood will be between the 

 pump and its outlets, and the blood pressure rises ; if, on 

 the other hand, the blood is passing rapidly through its 

 extensive arterioles into the veins the blood pressure falls. 

 When the amount poured into the venous system in any given 

 time, is equivalent to that pumped into the arterial system 

 during the same time (which is the perfectly normal con- 

 dition), we speak of the pressure as being a constant one. 



The above facts may be tabulated as follows : 



When the heart is more active the blood pressure rises. 



„ „ , less „ „ „ falls. 



When the arterioles contract the blood pressure rises. 



,, „ dilate „ „ falls. 



Nervous Mechanism. — The heart possesses no sensory 

 nerves ; it may be handled, pinched, pricked, or otherwise 



