NERVOUS CONTROL OF THE BLOOD-VESSELS 989 



and may possibly be ascribed in this case to abnormal irritations ascending the posterior 

 columns. 



The vaso-motor centre may also be directly affected by drugs such as digitalis or 

 strophanthus, both of which cause a rise in general blood-pressure from stimulation of 

 the centre. 



SPINAL CENTRES 



The great fall of blood-pressure observed after section of the cord in the 

 lower cervical region is not permanent. After one or two hours the pressure 

 begins to rise, and if the animal be kept alive, may attain a height only a 

 little inferior to that found in normal animals. 



If the spinal cord of such an animal be destroyed, the blood- pressure 

 sinks practically to zero and the circulation comes to an end, because the 

 animal has been, so to speak, bled to death into its own dilated blood-vessels. 

 In addition to the chief vaso-motor centre in the medulla there is a series 

 of subsidiary centres in the spinal cord, centres which we may probably 

 locate in the portions of grey matter situated in the lateral horns of the 

 cord and giving origin to the fibres which go to make up the white rami 

 communicantes. By means of these spinal centres a certain degree of 

 adaptation is possible between the blood- supply of the various parts of the 

 trunk. The important co-ordination between the state of the blood-vessels 

 and the condition of the central pump, the heart, is, however, wanting, since 

 the blood-vessels are now cut off from the cardiac centres and from the 

 part of the central nervous system which receives the afferent impulses 

 carried by the vagi. 



The spinal centres, like the chief vaso-motor centre, are susceptible to 

 changes in the composition of the blood supplied to them. If an animal be 

 kept alive by means of artificial respiration for a little time after division 

 of the cord just below the medulla, the blood-pressure slowly rises as the 

 spinal centres begin to take on their automatic functions. If artificial 

 respiration be now discontinued the asphyxia excites the centres of the cord. 

 The motor discharge to the skeletal muscles reveals itself in a single prolonged 

 spasm, since the respiratory centre is unable to take any part in directing the 

 motor discharges. Simultaneously with the spasm of the skeletal muscles 

 general constriction of the blood-vessels occurs which outlasts the muscular 

 spasms and causes a considerable rise of blood-pressure (Fig. 458). 



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 only obtained from the spinal animal 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 



