THE NERVOUS CONTROL OF THE BLOOD VESSELS 1033 



In this rise of pressure the main factor is lack of oxygen, and precisely 

 similar curves are obtained whether the asphyxia be produced by cessation 

 of artificial respiration or by administration of nitrogen. The same effect 

 may be produced by a very large excess of carbon dioxide, or by the injection 

 of acids into the circulation. There is a striking difference between the 

 sensibility of the spinal centres to these substances as compared with the 

 medullary centres. Thus the medullary vaso-motor centre is readily 

 excited by ventilation with 5 per cent, carbon dioxide, whereas a rise of 

 blood pressure is obtained from the spinal animal only when mixtures con- 

 taining 25 per cent, and upwards of carbon dioxide are employed. The 

 excitation of the medullary centre comes on about thirty seconds after the 

 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 

 the medulla, the common factor is probably increased H ion concentrati9n, 

 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 ascribethe 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 Be secured only by the 

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

 increase the state of constriction of the vessels; another set inhibitor or 

 dilator which will diminish the tone of the arteriole muscle and cause 

 vascular dilatation. Our knowledge of the existence of this second class 

 of nerve fibres to the vessels we owe also to Claude Bernard, who observed 

 that stimulation of the chorda tympani nerve not only evoked secretion 

 from the submaxillary gland but also increased the blood flow through its 

 vessels five or six fold. Subsequent researches have revealed the fact that 



