72 ELEMENTARY PHYSIOLOGY less. 



supply is usually the result of cutting off or lessening con- 

 strictor impulses which were previously passing along the 

 nerves to the arteries. But instances are met witli in tlio 

 body where the dilation is produced in an entirely different 

 way. Thus there is a certain nerve called the chorda 

 tympani, a branch of the facial or 7th cranial nerve (see 

 Lesson XL), which runs to the submaxillary salivary 

 glands. When this nerve is simply severed, no obvious 

 effect is produced on the blood-vessels of the gland. 

 But if now the cut end connected icilh the gland be sthau- 

 lated, the small arteries at once dihite powerfully, the 

 blood-supply is enormously increased, and the gland 

 becomes flushed. In this case we have to deal with 

 a vaso-motor nerve whose typical behaviour when 

 stimulated is, speaking broadly, the exact opposite to that 

 of the vaso-constrictor nerves. It is in fact a vaso-motor 

 nerve such that iin])ulses passing along it give rise not to 

 constriction but to dilation. Hence it is spoken of as a 

 vaso-dilator nerve. Other instances of the occurrence 

 of similar vasodilator nerves are met with, but as our 

 knowledge of them is at present uncertain and incomplete 

 we must be content with having simply drawn attention 

 to their existence, and to one striking instance of their 

 action. It will be obsei-ved that vaso-constrictor nerves 

 only lead to dilation through interference with the vaso- 

 motor centre and tonic inij)ulses ; vaso-dilator nerves 

 bring about dilatiuu ilircctly. 



17. Nervous Control of the Heart. Cardiac Nerves. 

 — The heart, as we all know, is not under the 

 direct influence of the will, but every one is no less 

 familiar with the fact that the actions of the heart are 

 wonderfully affected by all forms of emotion. Men and 

 women often faint, and have sometimes been killed by 

 sudden and violent joy or sorrow ; and when they faint 

 or die in this way, they do so because the perturbation 

 of the brain gives rise to a something which arrests 

 the heart as dead as you stop a stop-watch with a 



