298 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



discovered by stimulating certain nerve-roots in the upper thoracic 

 region ; but the action of vaso-motor nerves in the brain has 

 not yet been established by experimental evidence (see p. 278). 



The vaso-motor centre lies in the grey matter of the floor of the 

 fourth ventricle ; it is a few millimetres in length reaching from 

 the upper part of the floor to within about 4 mm. of the calamus 

 scriptorius. The position of this centre has been discovered by 

 the following means : when it is destroyed the tone of the small 

 vessels is no longer kept up, and in consequence there is a great 

 and universal fall in arterial blood-pressure ; when it is stimulated 

 there is an increase in the constriction of the arterioles all over 

 the body, and therefore a rise of arterial blood-pressure. Its upper 

 and lower limits have been accurately determined in the following 

 way ; a series of animals is taken and the central nervous system 

 divided in a different place in each ; the cerebrum and cerebellum 

 may be cut off without affecting blood-pressure, the vaso-motor 

 centre must therefore be below these ; if the section is made just 

 .above the medulla, the blood-pressure still remains high, and it 

 is not till the upper limit of the centre is passed that the blood- 

 pressure falls. Similarly in another series of animals, if the 

 cervical cord is cut through, and the animal kept alive by 

 artificial respiration, there is an enormous fall of pressure due to 

 the influence of the centre being removed from the vessels ; in 

 other experiments the section is made higher and higher, and the 

 same result noted, until at last the lower limit of the centre is 

 passed, and the fall of pressure is less and less marked the higher 

 one goes there, until in the animal in which the section is made 

 at the upper boundary of the centre the blood-pressure is not 

 affected at all, and the centre can be influenced reflexly by the 

 stimulation of afferent nerves, the pressor and depressor nerves, 

 which we shall be considering immediately. 



After the destruction of the vaso-motor centre in the bulb, 

 there is a fall of pressure. If the animal is kept alive, the 

 vessels after a time recover their tone, and the arterial pressure 

 rises : it rises still more on stimulating the central end of a sensory 

 nerve ; this is due to the existence of subsidiary vaso-motor centres 

 in the spinal cord ; for on the subsequent destruction of the spinal 

 cord the vessels again lose their tone and the blood-pressure sinks. 



The vaso-motor nerves travel down the lateral column of the 

 spinal cord, and terminate by arborising around the cells in the 

 grey matter of the subsidiary vaso-motor centres, the exact 

 anatomical position of which is uncertain. From these cells fresh 

 axis-cylinder processes originate which pass out as the small 

 medullated nerve-fibres in the anterior roots of the spinal nerves, 



