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THE POPULAR SCIENCE MONTHLY. 



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 liave 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 liv- 

 ing) portion hanging from the animal's back : not a single sound nerve- 



FiG. 2. . E, spinal marrow. N C, nerve-fllaraent distributed to the end of the tail. <?, its tro- 

 phic ganiflion. D N, a, nerve-fllament distributed to the skin of the back, lacerated in the 

 operation. G, its trophic j^anglion. C, the cicatrix which united the two nerves, and which 

 is now permeable to nervous shocks. S, point where the tail was cut in two ; a, b, arrows 

 showing the two directions in which sensory excitations are transmitted. 



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 frag- 

 ment 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 regener- 

 ated. 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-cica- 

 trix is : this is the reverse of the illusion observed in cases of ampu- 

 tation. 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. 



