6 LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



the same relative position in both the brain and spinal cord ; 

 that these fibres, when degenerated, are easily distinguished 

 from the surrounding healthy tissues, and so can readily be 

 followed up along their whole course. The study of this sec- 

 ondary degeneration has been of value in the field in which we 

 are now laboring. It promises still richer harvests in the future. 



The tract of fibres along which such a degeneration com- 

 monly spreads is called a system of fibres. A number of dis- 

 eases of the spinal cord, either at their beginning or throughout 

 their whole course, involve only certain systems of fibres; for 

 instance, the posterior columns of the cord. Such diseases are 

 called systemic diseases. The investigation of such systemic 

 diseases may also be of service in increasing our knowledge of 

 tbe course of fibres (Flechsig, Westphal, Striimpell). By the 

 accurate study of pathological changes Charcot and his pupils, 

 especially Pitres, Fere, Ballet, Brissaud, and others have done 

 fruitful work in the domain of brain anatomy. 



Occasionally structural abnormalities make it easier to 

 recognize one or another of the nerve-tracts than is the case 

 in the normal brain. Thus Onufrowics and Ivaufmann had the 

 opportunity to examine cases of defective corpus callosum in 

 which, owing to the absence of fibres of the latter, other bundles 

 of fibres appeared with a distinctness never before seen. 



It was a natural advance upon the study of these degenera- 

 tions to perform section of definite parts of nerve-roots or of 

 tbe spinal cord, for the purpose of studying the resulting 

 artificial secondary degenerations, and thus pressing farther 

 into the structure of the organs. Thus, for example, the sec- 

 tion experiments of Schiefferdecker and Singer very materially 

 increased our knowledge of the course of the roots of the spinal 

 nerves. 



If either peripheral or central nerve-substance is removed 

 from newborn animals, the fibres connected with the part re- 

 moved cease to develop and are gradually destroyed, appar- 

 ently because the development of the medullary sheath is 



