326 ACTION OP NERVES OF CAPILLARY VESSELS. 



the fibres which carry the impression to the nerve 

 centre are not those of the capillary vessels, but the 

 sensitive fibres distributed to the cutaneous surface. 

 It must, however, be remembered that there are no 

 special tactile organs in the part of the web experi- 

 mented upon, and .that the capillary vessels with 

 their nerve fibres lie immediately beneath the thin 

 cuticular covering of the web. 



The experiment is performed as follows : The foot 

 of a young living frog is well arranged for observa- 

 tion and covered with thin glass, so that the circula- 

 tion in the vessels can be well studied with a quarter 

 of an inch object-glass. The illumination must be 

 good. The tube of the microscope is then lengthened 

 by ten or twelve inches. By this proceeding we 

 greatly augment the magnifying power. Next, a 

 small artery is brought well into the field of the 

 microscope, and carefully focussed. While the ob- 

 server is looking intently at the artery the surface 

 of the web near the spot under examination is very 

 gently touched with the point of a needle, which 

 should have been already mounted in a handle made 

 of a piece of light wood.* Instantly the artery begins 

 to contract, and in a few seconds its cavity is so 

 narrowed that not a single blood corpuscle can pass. 

 The contraction is not quite even, for the outlines of 

 the vessel appear more or less wavy. After a few 

 seconds an undulatory movement of the coats is 

 noticed, and the artery gradually dilates to the same 

 degree as before the irritation of the web. 



The results of these experiments, considered in 

 connection with, the facts discovered by careful ana- 

 tomical investigation, prove, I think, conclusively the 

 function of the fine nerves distributed to the capil- 

 lary vessels, and indicate the precise manner in which 

 they act. These fibres must be afferent and carry 



* The experiment often succeeds when the most distant 

 parts of the web are touched. 



