Page:Popular Science Monthly Volume 11.djvu/243

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EXCITATIONS IN SENSORY NERVES.

there is no nervous apparatus of perception. But if we can bring this extremity into relation with the perceptive centre, the brain we shall see whether there is sensation, which would imply centrifugal propagation.

The means employed in bringing about this condition of things is very simple indeed. I remove, for the length of two or three centimetres, the skin from the tip of a young rat's tail, and insert the flayed part into the subcutaneous cellular tissue through an opening made in the skin of the animal's back. A few stitches suffice to hold the parts in place, and soon they adhere firmly, the rat's tail then having the ansate form.

PSM V11 D243 Nerve experiments with rats.jpg
Fig. 1.—Rat with Tip of Tail inserted into the Dorsal Tissues.

Eight months later I cut the tail in two, thus leaving two caudal stubs. Immediately after section, the dorsal stub was manifestly sensitive; and when it was pinched vigorously, the rat would squeal and run off. Hence it plainly appears that, in this fragment of a tail, the excitation of the sensor nerves is propagated from the thick to the slender end, or in a direction inverse to what is held to be the normal course. The process was about as follows: the sensor nerves distributed to the extremity of the tail, on being wounded by the removal of the skin, united with the sensor nerves of the dorsal region, which in like manner had been cut into. In due course of time the nerve-cicatrix became able to transmit the shocks (whatever their nature) which an excitation produces in a nerve. When, now, we pinch the extremity of the dorsal stub, the shock is transmitted along the excited caudal nerve, passes through the cicatrix, and follows the dorso-cutaneous nerve into the spinal marrow, which carries it to the brain, where it results in a sensation of pain.

The history of the case will be readily understood on examining the following diagram, in which, for simplicity's sake, I have represented only one of the nerve-filaments of the tail (N C) and one of the nerve-filaments of the back (D N).

But this sensibility of the dorsal fragment begins to grow less on