232 



and lessened flow through an organ, diminished volume and tension 

 of the organ ; the venous blood issues from it very slowly and is darker 

 in colour, and the temperature of the organ sinks. If a large area 



be constricted, the general arterial 

 pressure rises. 



The vaso-motor centre is situated 

 in the spinal bulb beneath the middle 

 of the floor of the fourth ventricle. 

 The tone of the vascular system is not 

 disturbed when the great brain and 

 mid-brain is destroyed as far as the 

 region of the pons, but as soon as the 

 spinal bulb is injured or destroyed the 

 arterial pressure falls very greatly, and 

 the animal passes into a condition of 

 surgical shock if kept alive by artificial 

 respiration. Painting the floor of the 

 fourth ventricle with a local anaesthetic 

 e.g., cocaine has the same lowering 

 effect on the blood-pressure. Division 

 of the cervical spinal cord or of the 

 splanchnic nerves lowers the blood - 

 pressure greatly. The one lesion cuts 

 off the whole body, the other the 



abdominal organs, from the tonic in- 



f/f fluence of the centre. The fall of 



" pressure is due almost entirely to the 



pooling of the blood in the portal veins 

 and vena cava inferior. On the other 

 hand, electrical excitation of the lower 

 end of the divided cord or splanchnic 

 nerves raises the pressure by restoring 

 the vascular tone. If an animal be 

 kept alive after division of the spinal 

 cord in the lower cervical region, which 

 is possible, since the phrenics, the chief 

 C, Constrictor; D, dilator neurones; motor nerves of respiration, come off 

 A, inuscle cell of artcriole in body ; above this region, it is found that the 

 K, muscle cell of arteriole of vascular tone after a time becomes 

 kidney; It, afferent nerve of the ,,, -..,. *i_i 



kidney influencing the body restored and the condition of .shock 

 arteriole through the bulbar passes away. By no second section 

 centres and the kidney arteriole o f tne gp^a! cor( J can t he general 

 locally through the spinal centres. , ... , , , , , 



Effect on centres shown by + and condition of shock be reproduced, but 

 - signs. (Bayliss.) a total destruction of the cord once 



more causes a general loss of the 



vascular tone. From the experimental result so obtained, it is 

 argued that subsidiary vaso-motor centres exist in the spinal cord, 

 and there is evidence to show that these centres may be excited re- 

 flexly. After the lumbar cord has been destroyed, the tone of the 



FIG. 123. 



