254 THE HUMAN BODY, 



keeping the arteries more constricted than they would be 

 under the influence of their intrinsic nerves alone. Accord- 

 ingly if they are cut, or paralyzed, in any region of the Body 

 its arteries dilate and it becomes flushed with blood. Those 

 of the external ear, for example, run in the cervical sympa- 

 thetic, from the lower part of the neck where they leave the 

 spinal cord, until they reach the arterial branches for the 

 ear and run along the smaller twigs to it. If, therefore, 

 the cervical sympathetic be divided on one side in an 

 anaesthetized rabbit, the ear on that side becomes red and 

 warm from the dilatation of its arteries and the extra 

 amount of blood flowing through it. If, however; that end 

 of the cut nerve still attached to the ear be excited electri- 

 cally or otherwise, the ear arteries contract gradually until 

 their passage is almost closed up, and the whole organ be- 

 comes cold and very pale. Although these vaso-constrictor 

 fibres are thus shown to pass through the cervical sympa- 

 thetic, other experiments show that they really originate 

 in a group of nerve-cells in the medulla oblongata, and 

 from there run down the spinal cord to the lower part of 

 the neck, where they pass out in the anterior roots of some 

 spinal nerves and reach the sympathetic syste^m. The same 

 is true of nearly all extrinsic vaso-constrictor nerve-fibres 

 in the Body. Some few possibly arise from centres -in the 

 spinal cord, but the great majority come primarily from 

 the medulla oblongata, and the collection of nerve-cells 

 there from which they spring is known as the vaso-motor 

 centre; a better name would be the vaso-constrictor centre. 

 The Control of the Vaso-Motor Centre. The vaso- 

 motor centre is automatic; that is to say it maintains 'a 

 certain amount of activity of its own, independently of any 

 stimuli reaching it through afferent nerve-fibres. Never- 

 theless, like nearly all automatic nerve-centres, it is under 

 reflex control, so that its activity may be increased or les- 

 sened by afferent impulses conveyed to it. Nearly every sen- 

 sory nerve of the Body is in connection with it; any stimu- 

 lus giving risp f.f) pn.in, for example, excites it, and thus 

 constricting the arteries, increases the peripheral resist- 

 ance to the blood-flow and raises arterial pressure. On 



