246 HANDBOOK OF PHYSIOLOGY. 



nerves of the brachial plexus: when these are divided, dilatation of the 

 blood-vessels in the parts supplied by them takes place. It appears, 

 therefore, that nerves exist which have a distinct control over the vas- 

 cular supply of every part of the body. These nerves are called vaso- 

 motor. 



Recently Mall has shown that veins possess a vaso-motor nerve-supply 

 as well as arteries. 



Vaso-motor nerves may be divided into two classes, according to their 

 function of causing contraction or dilatation of the blood-vessels, into. 

 vaso-constrictor and vaso-dilator nerves. 



Vaso-motor Centres. Bulbar Centre. The bulbar vaso-con- 

 strictor centre in the rabbit lies in the floor of the fourth ventricle, a 

 millimetre or two caudal to the corpora quadrigemina, and extends, 

 longitudinally over an area of about 3 millimetres. Owsjanuikow has 

 shown that the centre is composed of two halves, each half lying in the 

 lateral column to the side of the median line. This centre is in con- 

 stant action, as is shown by dilatation of the blood-vessels when removed 

 from its action by section of the spinal cord. 



The existence of a vaso-dilator centre in the spinal bulb has not been, 

 proved. 



Spinal Centres. Secondary vaso-motor centres are present in tha 

 spinal cord (Goltz). Under normal conditions they do not act indepen- 

 dently of the bulbar centre^ but when the action of the latter has been 

 interrupted by section of the cord, certain spinal cells below the section 

 take on central functions and bring about a re-establishment of the lost 

 vascular tone. Moreover, the central functions disappear if the cord 

 below the section be destroyed. 



Sympathetic Vaso-motor Centres. The existence of sympathetic 

 vaso-motor centres has been proved by the experiments of Goltz and 

 Evvald. It was found by these observers that even after destruction of 

 the lower part of the spinal cord, the tone of the vessels of the hind 

 limbs, lost as a result of the operation, was re-established later. 



Vaso-motor Reflexes. The secondary vaso-motor centres, when 

 removed from the influence of the bulbar centre, respond to afferent im- 

 pulses by vaso-motor action. But under normal conditions the bulbar 

 centre controls vaso-motor reflexes. The afferent impulses which excite' 

 reflex vaso-motor action may proceed from the terminations of -sensory 

 nerves in general or from the blood-vessels themselves, and the constric- 

 tion or dilatation which follows generally occurs in the area whence the 

 impulses arise. Yet the reflex may appear elsewhere, e.g., the vessels of 

 the submaxillary gland dilate when the tongue is stimulated an associa- 

 tion in function. Impulses proceeding to the vaso-motor centre from the 



