408 FUNCTIONS OF THE NERVES. [ch. hi- 



communicates the impression to the mind ; and the nerve pro- 

 duces motion by means of a muscle, when an impression made 

 on the nerve descends to the muscle/ and excites it to move- 

 ment. Consequently, a nerve separated from the brain, and no 

 longer able to communicate impressions to it, can no longer 

 produce sensation, just as a nerve separated by division from a 

 muscle can no longer excite motion in the muscle, however 

 much it may be irritated. Consequently, a nerve has a similar 

 office in exciting sensation and motion, namely, to receive the 

 impression of a stimulus, and to transmit it with the greatest 

 rapidity along its whole length, which, when it arrives at the 

 brain, produces the perception of a sensation, but when it 

 arrives at a muscle, excites its contraction. 



SECTION II. THE ACTION OF THE NERVES ON THE VESSELS AND 



THEIR FLUIDS. 



Another function of the nerves consists in a certain power 

 over the blood-vessels, and specially the capillaries, in virtue 

 of which, when the nerves are stimulated, they excite in that 

 part to which they are distributed a much more copious 

 accumulation of blood than would have taken place in the 

 normal condition of the circulation. This phenomenon is 

 termed congestion of the humours, afflux, derivation, abnormal 

 direction, descent of the humours. Stahl termed it the tide of 

 the microcosmic sea, or the ebb and flow of the blood. 



The causes that determine a more copious derivation of the 

 humours into any part of the body, are usually considered to be 

 twofold ; the one, a mere mechanical cause, consists in a dimi- 

 nished resistance of the vessels of the part, so that the humours 

 contained in the vessels being forced on by the power of the 

 heart and the vessels themselves, flow to the point of least 

 resistance, according to the laws regulating other fluids, and 

 cause congestion of the humours ; for this reason, when a vein 

 or artery is divided, the blood rushes from the adjoining vessels, 

 even against its natural direction and gravity; for this reason, 

 also, congestion takes place, when vessels are relaxed by emol- 

 lient cataplasms and pediluvia.^ Thus also the blood is cou- 



' Haller, De Part. Corp. Hum. Fabr., torn, iv, pp. 93, 289. 



