SECT. II.] ACTION OF THE NERVES ON THE VESSELS. 409 



gested under a cupping-glass when the usual atmospheric pres- 

 sure is removed from the part ; a local derivation of the humours 

 takes place also when a compression of the vessels occurs in any 

 part, and the blood is repelled into other parts ; as occurs, for 

 example, when the stomach is filled, by which the abdominal 

 vessels are compressed and the lungs forced into less room, 

 and, consequently, a greater quantity of blood goes to the head, 

 rendering the face redder. The other cause is a stimulus to 

 the nerves ; which when applied to the nerves excites a more 

 copious flow of humours. Innumerable phenomena of daily 

 occurrence show this. Thus a stimulus applied to the nerves is 

 the cause why the cheeks, ears, and nose, become intensely red, 

 and a sense of heat is felt when exposed to a cold wind in winter. 

 No one is ignorant how much the stimulus of sinapisms and 

 blisters cause derivation of the humours to the stimulated part; 

 an acrid smoke or fine powder getting into the eyes excites a 

 copious flow of tears, and the vessels of the conjunctiva, pre- 

 viously invisible, become distended with blood. The smoke of 

 tobacco, or any other acrid aroma, retained in the mouth, 

 excites a copious flow of saliva ; purgatives and emetics bring 

 off* much gastric and intestinal mucus ; titillation of the nipple 

 of the mamma causes it to become turgid and erect ; the touch, 

 or the stimulus of the semen or urine, or a gonorrhoea, cause 

 the penis to be distended and erected by exciting a more copious 

 flow of blood into the corpora cavernosa. These phenomena 

 take place if the nerves be stimulated locally ; but the same 

 thing happens when the nerves are excited not directly, but 

 indirectly, through the brain. We know, that thus the face is 

 suffused with the blush of modesty ; grief causes a copious flow 

 of tears, congestion of the vessels of the conjunctiva, and 

 redness and swelling of the whole face. The sight of agree- 

 able food provokes a flow of saliva ; it is not unusual for some 

 persons to vomit, or be purged by only seeing a medicine ; a 

 lascivious idea erects the penis, &c. 



Although it is placed beyond a doubt, that stimulated 

 nerves cause congestion and derivation of the humours to the 

 part stimulated, the mode in which the nerves accomplish this is 

 as yet unknown. Distinguished writers have advanced various 

 conjectures, by which they have attempted to explain this in- 

 fluence of the nerves on the vessels. Some have supposed that 



