1112 PHYSIOLOGY 



constrictions of the blood-vessels occur which, outlast the muscular 

 spasms and cause a considerable rise of blood pressure (Fig. 461). 



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

 precisely similar curves are obtained whether the asphyxia be pro- 

 duced 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. 



FIG. 461. 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 dis- 

 continued . 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 



Thus the medullary vaso-motor centre is readily excited by 5 per cent, 

 carbon dioxide, whereas a rise of blood pressure is only obtained from 

 the spinal animal when mixtures containing 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 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 concentra- 

 tion, 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. 



