86 THE INVOLUNTARY NERVOUS SYSTEM 



independent of the organs it supplies, but rather as a handmaid of 

 such organs, supplying more or less blood to them as they require, 

 and therefore that the regulation of the calibre of the blood vessels 

 might be brought about otherwise than by the action of inhibitory 

 nerve cells. At the present day it is recognized that the meta- 

 bolism of an organ may be affected by the activity of another 

 organ even at a distance, without the intervention of the nervous 

 system, by means of chemical factors conveyed from the one 

 organ to the other by the circulation; the classical example of 

 such chemical messengers or hormones is the passage into the 

 circulation of secretin from the duodenal mucous membrane to 

 bring about the activity of the pancreatic gland. In any case 

 of vascular dilatation we must always bear in mind the pos- 

 sibility of a direct chemical action upon the small blood vessels 

 by the products of metabolism of the organ rather than an ' in- 

 hibition ' by inhibitory nerves, and it is possible that the main 

 factor in the regulation of the blood supply to an organ may be 

 chemical rather than nervous. 



But in addition to ensuring a greater supply of blood to an 

 organ, when that organ is in activity, the vascular system requires 

 regulation in its relation to the central organ, the heart, the effici- 

 ency of which is dependent so intimately upon the blood pres- 

 sure in the vascular system ; such a regulation is essentially of a 

 nervous nature and makes it probable that the vascular muscles 

 like the other muscles of the involuntary system possess inhibi- 

 tory as well as motor nerves. 



The first evidence for the existence of nerves which dilated 

 blood vessels was given by Claude Bernard, who found that 

 stimulation of the chorda tympani caused not only secretion of 

 the submaxillary gland but also dilatation of its blood vessels. 

 Then came the observation of Eckhard upon the meaning of 

 erection, in which he showed that, when erection was caused by 

 stimulation of the nervus erigens, there was a marked dilatation 

 of the blood vessels of the penis. Vulpian showed the redden- 

 ing of the tongue upon stimulation of the lingual nerve and 

 proved that these dilator fibres came from the chorda tympani. 

 Later Loeb and Eckhard showed that the tympanic branch 

 of the glosso-pharyngeal behaves to the parotid gland and its 

 blood vessels in the same manner as the chorda tympani to the 

 submaxillary gland and its blood vessels. As early as 1869 



