IV] THE BLOOD AXD THE ORGANS OF CIRCFLATION 65 



breathing results in a suction of blood along the large veins 

 entering the heart, and the compression of the diaphragm assists 

 in driving the blood in the same direction. ^loreover, the move- 

 ments of the muscles involved in exercise exert pressure on the 

 veins, and tend to drive the blood onwards, the action of the 

 valves assisting. In the same khid of w ay muscular movements 

 cause the lymph to flow along the lymphatic vessels which are 

 also provided with valves. It is parth' for this reason that 

 exercise has a beneficial effect upon the circulation, and it is well 

 known that horses whose legs have become swollen or oedematous 

 through much standing may often be cured by moderate exercise 



Blood Pressure. It is of course obvious that the blood is 

 retained in the body under pressure. Thus, if we cut a large 

 artery the blood flows out in a forcible stream and with spurts 

 correspondmg to the heart beats. Exact measurements of the 

 degrees of pressure were first obtained by Hales who connected 

 the femoral artery of a horse with a long glass tube in which the 

 blood rose to a height of eight feet, three inches and then remained 

 approximately steady. This experiment showed that in the 

 normal closed artery the blood was under a pressure or tension 

 sufficient to support the weight of a column of blood (or water) 

 to. this height. He then performed a. similar experiment with a 

 large vein and found that the blood rose only to a height of one 

 foot, showing that the blood pressure m the veins is very much 

 less than in the arteries. Later experimenters have substituted 

 a column of mercurj' for a column of blood, mercury being 

 13'5 times as heavy as blood (or water). Moreover, a manometer 

 formed in this way maj^ have its piston connected with a recording 

 drum, and a permanent tracing of the variations in the blood 

 pressure can be obtained. 



The degree of arterial pressure is not due solely to the heart ; 

 it is also regulated by the action of the nerves supplying the 

 arteries. These nerves are of two kinds. There are vaso- 

 constrictor fibres belonging to the sympathetic sj^stem (on which 

 the hormone adrenalin acts), and vaso-dilator fibres of different 

 and diverse origin, which produce inhibition or temjDorary 

 paralysis of the arterial muscles, and so cause the arteries to 

 expand. 



