The Bloodvessels. 71 



instance, in the sinuses of the brain and in the veins of 

 the foot ; in the latter case the movements occurring in the 

 wall, the result of expansion and contraction, are amply 

 sufficient to assist in the immense venous circulation of 

 the part. 



Influence of the Nervous System. — When a part requires 

 more blood— for example, an actively secreting gland — the 

 vessels leading to it dilate ; when the necessity for the 

 increase has passed, the vessels contract. This dilatation 

 and contraction of the bloodvessels is under the control 

 of the nervous system, through the minute nerves known 

 as vaso-motor distributed to the muscular walls of the 

 small arteries, and it is possible that the branches of nerve 

 which dilate the vessels are not those which constrict them. 



The nerves which contract the vessels are called vaso- 

 constrictor, whilst those which dilate the vessels are called 

 vaso-dilator : it is believed that in most cases dilatation of 

 the vessels is due to diminished constricting influences, 

 rather than to special dilator fibres. 



If the sympathetic nerve in the neck of a horse be 

 divided, the temperature of the face and neck on the same 

 side rises, the skin becomes congested and sweating occurs. 

 In other words, the system of nerves which constrict the 

 bloodvessels is paralysed by division, and dilatation results. 

 If now the cut end of the nerve be irritated, the temperature 

 of the part falls, the skin congestion is removed, and 

 sweating ceases ; the constrictor fibres are now active, and 

 the amount of blood sent to the part is considerably re- 

 duced. 



The above experiment is best made on the rabbit, where 

 the vessels may be seen dilating and the skin reddening. 



The vaso-motor fibres may run in a sympathetic or a 

 spinal nerve. The sciatic nerve, for example, contains the 

 vaso-motor fibres for the posterior extremity, whilst the 

 sympathetic system contains similar fibres for the abdominal 

 viscera. It is said that the vaso-dilator nerves are principally 

 derived from the cerebro-spinal system, whilst the con- 

 strictors belong to the sympathetic. 



