NERVOUS CONTROL OF THE BLOOD-VESSELS 1113 



THE PERIPHERAL TONE AND ADAPTATION OF THE 

 BLOOD-VESSELS 



Division of the sciatic nerve causes an immediate dilatation of the 

 vessels of the lower limbs in consequence of their severance from the 

 tonic activity of the vaso-motor centres. This dilatation passes off 

 in a day or two and the vessels acquire a tone, i.e. remain in a state of 

 average constriction which can be increased or diminished by locrl 

 conditions. This recovery of tone has been ascribed by many physio- 



Leg 



volume 



Spl. exc. 



T FIG. 462. Effect of excitation of splanchnic nerves on the blood pressure and 

 on the volume of the denervated hind limb of the cat. (BAYLISS.) 



legists to the existence of a third set of nerve-centres in the walls of the 

 arteries. In the absence of any direct histological evidence of the exist- 

 ence of such centres it seems more rational to ascribe the tonus to the 

 automatic activity of the muscular fibres themselves. Like all muscu- 

 lar tissues, the arterial wall after severance from all its nervous connec- 

 tions is largely influenced by tension, increased tension acting as a 

 stimulus to increased contraction. This effect may be observed in 

 the isolated arteries. A strip of the carotid, even a day or two after 

 death, if warmed, may respond to a sudden distending force by a slow 

 contraction. The same response may be observed in denervated 

 blood-vessels through which the circulation is well maintained. Thus, 

 as Bayliss has shown, if the hind limb, after division of all its nerves, 

 be placed in a plethysmograph, a sudden rise of general blood pressure, 

 such as that produced by stimulation of the splanchnic nerves, may 

 cause an immediate passive dilatation, which is rapidly followed by an 



