Page:Popular Science Monthly Volume 11.djvu/244

232 the second day after section, and soon it is gone. If, some days later, we examine the nerves of this fragment with a microscope, we shall find that they have undergone the changes usual in nerves that have been cut off from their trophic centres; and this is true no less of the portion concealed under the skin than of the free (and certainly living) portion hanging from the animal's back: not a single sound nerve-



tube is to be seen in it. On the contrary, the stump of the tail, which has retained its natural position, has every nerve sound, with not a single diseased tube.

Thus, then, physiological facts are in agreement with histological observations, and they both prove conclusively that the sensor nerves which transmitted the centrifugal excitation were the normal nerves of the dorsal stub, and that here we have neither new-formed nerves nor nerve-fibres with ansate terminations. It is further demonstrated—and this is a fact not without interest—that the relations with the nerve-centres of perception, from which results sensation, are more easily established than those with the trophic ganglionic centres, which nourish the sensor nerves. Who knows but that, had I waited longer before I cut the tail in two, the influence of the new trophic centres would have become sufficient to maintain the nerves of the dorsal fragment in their integrity, and that sensibility would have persisted after section? Some months after the dorsal stub had become insensible, it again regained sensibility, the diseased nerves having been regenerated. At first, just as I had observed in 1863, the animal refers the impression it receives to the region of the back where the nerve-cicatrix is: this is the reverse of the illusion observed in cases of amputation. By little and little the rat is educated, and at last recognizes the exact point that is excited; thus showing that our so-called innate knowledge of the place occupied in space by every point of our bodies is, like all our knowledge, merely the result of repeated experiences.