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 

 ia 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 local 

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



Leg 

 volume 



B.P. 



Spl. exc. 



Time 

 10 sec. 



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.) 



logists 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 



