HISTORY AND METHODS OF INVESTIGATION. 7 



point where the nerve is severed, and that the cell may be temporarily 

 much damaged, at any rate structurally modified. In such cases the de- 

 generation of the central segment of the axis-cylinder also supervenes, 

 simply because it is not in connection with a normal parent-cell. In the 

 consideration of secondary degenerations one must, in future, give due 

 weight to these facts, which are of especial importance pathologically. 



The region through which such a degeneration is always propagated 

 is called a tract {Faser system). A number of diseases of the spinal cord, 

 either in their inception or throughout their course, affect particular tracts: 

 e.g., only the posterior tracts of the spinal cord. Such diseases are called 

 "System-diseases." The study of such diseases can be utilized for the 

 increase of our knowledge of the, course of nerve-tracts (Flechsig, Westphal, 

 Striimpell). Further, through an exact study of pathological changes 

 Charcot and his pupils — especially Pitres, Fere, Belief, Brissaud, et al — 

 have added much to our knowledge of brain-anatomy. 



Occasionally malformations make it possible to differentiate one tract 

 from another more readily than in the normal brain. Thus Kaufmann 

 and others were able to study cases of absence of the corpus eallosum in 

 which, just because of the absence of the commissure in question, other 

 features of the brain came out into previously unknown prominence. These 

 observations suggested the experimental severing of particular parts of 

 the roots or of the spinal cord, and, through the intentionally induced 

 secondary degeneration, to gain further knowledge of the structure. 

 Numerous experiments of this kind were made, and for many important 

 facts we have to thank those experimenters who proceeded in this way. For 

 example, through the nerve-cutting experiments of Singer and of Schieffer- 

 decker and later of Lowenthal, Sherrington, Mott, et al, our knowledge of 

 the course of nerve-roots in the spinal cord has been much enriched. 



Such degenerations may be studied according to one or the other of 

 two methods: One may either wait for the complete destruction of the fibers 

 and then follow the course of the atrophied tract, or one may, within a 

 few weeks after the operation, treat a preparation with a solution of osmic 

 acid, which blackens the products of degeneration (Marchi). The last 

 method especially gives very clear pictures, showing the degenerated fiber? 

 in lines of black points upon a clear field. 



If, in a newborn animal, peripheral or central nerve-substance be re- 

 moved, fibers involved in the injury or operation do not further develop, 

 but, in fact, gradually, yet completely, degenerate. 



Gudden (1870) used this fact to furnish us with a new and fruitful 

 method of investigation. For example, after extirpation of an eye he fol- 

 lowed, through means of sections, the atrophy taking place in the brain, 

 and so found the central ending of the optic nerve in question. On what- 



