THE VASO-MOTOR NERVES 



231 



The vase-motor centre exerts a tonic influence over the calibre 

 both of the arterial and portal systems. 



Much work has been done to determine the origin and exact 

 distribution of the vaso-motor nerves to the various organs, and 

 the reflex conditions under which they come normally into action, 

 and our knowledge, the fruits of these inquiries, has come to a con- 

 dition of considerable exactness. This knowledge is of great practical 

 importance to the physician, and it has been obtained entirely by 

 experiment on living anaesthetized animals. No dissections of the 

 dead animal could have informed us of the vaso-motor nerves. Vaso- 



C.C. 



FIG. 122. 



F, Depressor; R, pressor afferent impulses affecting the arteriole muscle through CC, 

 vaso -constrictor centre, and through DC, vaso-dilator centre. Effect shown by 

 + and signs. (Bayliss.) 



motor effects can be studied by ( 1 ) observing the flushing or blanching 

 of an organ; (2) measuring the temperature of a part or organ; (3) 

 measuring the venous outflow ; (4) recording the pressure in the artery 

 going to and the vein leaving the organ; (5) observations on the 

 volume of an organ. To make these last observations, the organ is 

 enclosed in a suitable air-tight box, or plethysmograph, an opening 

 being contrived for the vessels of the organ to pass through so that 

 the circulation may continue. The box is filled with air or water, 

 and is connected with a recording tambour (Fig. 120). 



The chief effects of vaso -constriction are an increased resistance 



