58 The Nerves of Capillary Vessels and their 
of the mucous membrane. But there are certain physiological ex- 
periments which have a very important bearing upon the question 
under discussion, and to these I will now direct attention. 
Physiological Experiments. 
Many years ago, when I possessed two living specimens of the 
Proteus, which I brought home from the cave at Adelsberg, I often 
observed the change which instantly took place when a bright ray 
of light was suddenly thrown upon the vessels of the exposed 
branchiae. The little arteries of the gill suddenly contracted, and 
the entire volume of the vascular tuft was reduced by at least one- 
third. The circulation through the vessels was instantly retarded, 
the diameter of the capillaries was sensibly reduced, and the flow of 
the blood stream distinctly checked. The ray of light, I conclude, 
acted directly upon sensitive nerve fibres distributed around the 
capillaries, which carried an impression to the nerve centre, and 
through the vaso-motor nerves which were connected with this centre 
the muscular fibre cells of the artery were made to contract. 
Another experiment which may be easily tried upon the web 
of the foot of the living frog is almost as conclusive, although it is 
open to the objection that the fibres which carry the impression to 
the nervous 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 experimented 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 observation and covered with thin 
glass, so that the circulation 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 eight or nine 
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 observer 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 experiment often succeeds when the most distant parts of the web are 
touched. 
