6 NERVOUS SYSTEM OF VERTEBRATES. 



Stain of certain plastic materials within nerve elements which 

 are doubtless either nutritive or excretory in their nature, or both. 

 These materials constitute the so-called Nissl bodies whose form 

 and volume change with changing physiological states of the nerve 

 elements. The study of these elements is therefore of great 

 importance to the physiologist and the pathologist. 



The other method includes a considerable number of proced- 

 ures which may be spoken of collectively as neuro-fibrillar staining 

 methods. These stains affect the more stable colloidal substances 

 in nerve elements and so are complementary to the Nissl method. 

 Besides the methylene blue stain mentioned above a number of 

 methods have been used for this purpose, of which the latest are 

 certain modifications of the photographic procedure introduced 

 by Cajal, Bielschowsky and others. 



The methods thus far mentioned are anatomical methods 

 chiefly applicable to the adult nervous system. When the nervous 

 system is studied with especial reference to its development or 

 by making use of any pecuHar characters which it possesses 

 during development, it is proper to speak of embr\^ological methods. 

 These include the study of the de^■elopment of the nen'ous system 

 by any of the procedures applicable to embrj'os and the study of 

 histogenesis by various staining techniques, including those of 

 Golgi and of Ehrlich mentioned above. A special embryological 

 method of great importance, the method of Flechsig, consists 

 in the study of the course and order of myelination of nerve tracts 

 by means of the Weigert technique. This gives especially favorable 

 opportunities for studying the course of fiber tracts because 

 specific tracts can be studied at the time of appearance of their 

 myelin sheaths, certain tracts frequently presenting themselves 

 in entire isolation from others. 



Experimental methods. — The anatomical study of the nervous 

 system may be greatly extended by means of experiments which 

 produce artificial differences between nerve tracts or nerve centers, 

 which may then be brought to view by suitable staining. Most 

 prominent among these methods is the study of fiber tracts which 

 have been caused to degenerate in accordance with Waller's 

 law. Negative pictures of such tracts are obtained when the 



