THE VASCULAR MECHANISM. 227 



communicantes to form part of the sympathetic system ; their subse- 

 quent course has already been described (p. 101). 



The mean arterial pressure is determined largely by the resistance 

 offered by the arterioles to the flow of blood through them ; it rises 

 when they constrict, and falls when they dilate. Hence stimulation of 

 the vaso-motor centre by causing constriction of arterioles all over the 

 body produces an enormous rise of blood pressure ; destruction of the 

 centre is followed by dilatation of the arterioles, and the blood pressure 

 falls to 40 mm. Hg or less. The centre lies in the floor of the fourth 

 ventricle, its lower limit in the rabbit being about 4 mm. above the 

 apex of the calamus scriptorius, and its upper limit about 4 mm. higher. 

 Its position has been ascertained experimentally by observing the effect 

 on the blood pressure of transection of the brain-stem at various levels. 

 Section through the pons or upper part of the medulla oblongata does 

 not affect the blood pressure; when the section passes through the 

 upper end of the centre it produces a slight fall of pressure, and if a 

 section is made a few millimetres lower the fall of pressure is maximal. 

 On division of the spinal cord in the cervical region, all the arterioles 

 are cut off from the vaso-motor centre, and the fall of blood pressure is 

 as great as after destruction of the centre itself. When the transection 

 is made in the dorsal region, only those arterioles which receive vaso- 

 constrictor nerves from the spinal cord below the lesion will lose their 

 tone ; and the fall of arterial pressure becomes less marked the lower 

 the level at which the spinal cord is divided, transection in the lower 

 lumbar region having no effect upon the mean arterial pressure. 



If an animal is kept alive for some hours or days after transection 

 of the spinal cord, its arterioles gradually recover their tone and the 

 blood pressure returns to a normal level, This is brought about by 

 means of subsidiary vaso-motor centres in the spinal cord, which are 

 called into play when the medullary centre is put out of action. On 

 subsequent destruction of the spinal cord, the blood pressure falls almost 

 to zero. 



The only arterioles in the body which are not known to be influenced 

 by vaso-constrictor nerves are the cerebral and coronary vessels. Re- 

 cent observation has shown that, contrary to the views formerly held, 

 vaso-motor fibres are distributed to the pulmonary vessels ; their exist- 

 ence has been demonstrated by means of adrenalin (p. 234), which stimu- 

 lates the endings of the vaso-constrictor fibres and produces constriction 

 of the arterioles. The addition of adrenalin to the blood flowing through 

 the lungs is followed by constriction of the pulmonary arterioles and a 

 rise of pressure in the pulmonary artery, and if the lungs are perfused 

 under a constant pressure the outflow of blood is diminished. 



