990 PHYSIOLOGY 



administration of nitrogen has commenced in contrast to that of the spinal 

 centres which does not occur until two minutes or more have elapsed. In 

 the intact animal a maximal stimulation of the vaso-motor centre is pro- 

 duced in the cat by the injection of 2 c.c. N/20 lactic acid, whereas 5 c.c. 

 of N/2 acid are required to excite spinal cord centres. Here therefore, as in 



FIG. 458. Blood-pressure tracing taken by a mercurial manometer from carotid 

 artery of a dog, three hours after section of the cord, just below the medulla 

 oblongata. At o the artificial respiration was discontinued. A general spasm 

 of the skeletal muscles occurred between x and x. The muscles then relaxed, 

 and were flaccid during the rest of the rise of blood-pressure. 



the medulla, the common factor is probably increased H ion concentration, 

 the excitation threshold for the medullary centres being only about one- 

 fifth that of the spinal centres. 



The local spinal centres are connected with the medullary vaso-motor 

 centre on each side by tracts of nerve fibres which descend in the lateral 

 columns of the cord. 



THE PERIPHERAL TONE 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 constric- 

 tion which can be increased or diminished by local conditions. This recovery 

 of tone has been ascribed by many physiologists 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 existence of such centres it seems more 

 rational to ascribe the tonus to the automatic activity of the muscular fibres 

 themselves. 



THE COURSE AND DISTRIBUTION OF THE VASO-MOTOR NERVES 



Since the blood-vessels, like the heart, are the seat of an automatic 

 activity, complete nervous control of these tubes can only be secured by the 

 provision of two sets of nerves : one set augmentor or motor which will 



