296 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



is less than the mean arterial pressure, the artery will not be 

 compressed to its utmost during diastole ; if, on the other hand, 

 the pressure exerted is greater than the mean, the artery will not 

 fully expand during systole. In either case, the pulsation will not 

 be so great as when the pressure exerted on the outside of the 

 artery equals the mean pressure within. 



By recording the arterial pressure in the dog with a mercury 

 manometer, and at the same time taking readings with the 

 sphygmometer, the instrument has been proved to give accurate 

 results. 



The normal pressure in the radial artery of healthy young adults 

 is no to 1 20 mm. Hg. It appears to be as constant as the body 

 temperature. In the recumbent posture the pressure is slightly 

 lower than in the erect position. This relation is reversed in con- 

 ditions of exhaustion. During muscular exertion the pressure 

 is raised, while in the subsequent period of rest it is sub-normal. 

 Mental work raises the pressure ; during rest and sleep it is 

 lowered. The taking of food produces no noteworthy effect. 

 In disease there are naturally variations in different directions, 

 and the study of these has already yielded valuable results. 



With this instrument the venous pressure can also be obtained 

 in the manner suggested by Dr. George Oliver. On the back of 

 the hand or arm a vein is chosen free from anastomoses, and the 

 sphygmometer is pressed upon the peripheral end of this. The 

 vein is then emptied centrally i.e., towards the heart by the 

 pressure of the finger. Next the pressure in the sphygmometer 

 is gradually relaxed, and the exact height noted at which the vein 

 refills with blood. 



Since the flow of blood through the capillaries is maintained 

 by the difference in pressure between the artery and vein, we can, 

 by obtaining readings both of the arterial and of the venous 

 pressures, estimate the comparative efficiency of the capillary 

 circulation in man under varying conditions. 



The Vase-motor Nervous System. 



The vaso-motor nervous system consists of the vaso-motor centre 

 situated in the bulb, of certain subsidiary vaso-motor centres in 

 the spinal cord, and of vaso-motor nerves, which are of two kinds 

 (a) those the stimulation of which causes constriction of the 

 vessels ; these are called vaso-constrictor nerves ; (b) those the 

 stimulation of which causes dilatation of the vessels; these are 

 called vaso-dilatator nerves. 



The following names are associated with the history of the 



