40 SPECIAL SENSES. 



either side to the eye of the opposite side. The greatest 

 part of the fibres take this direction. Their relative situa- 

 tion is internal. 



2. External fibres, much less numerous than the preceding, 

 which pass from the optic tract to the eye upon the same 

 side. 



3. Fibres, situated on the posterior boundary of the com- 

 missure, which pass from one optic tract to the other and do 

 not go to the eyes. These fibres are scanty, and, according 

 to Sappey, are sometimes wanting. 1 



4. Fibres, situated on the anterior border of the commis- 

 sure, more numerous than the preceding, which pass from 

 one eye to the other, and which have no connection with the 

 optic tracts. 2 



It is probable, reasoning chiefly from cases of cerebral 

 injury or disease, that the filaments from the optic tracts 

 upon the two sides are connected with distinct portions of 



1 SAPPEY, Traite d* anatomic, Paris, 1871, tome iii., p. 254. 



2 Within the last few years, the old idea that the optic nerves make a com- 

 plete decussation at the commissure has been revived. In two very elaborate 

 articles on this subject, which have just appeared (1873) in Graefe's Archiv, by 

 Mandelstamm and by Michel, the authors, after referring to the anatomical re- 

 searches of Biesiadecki, in 1861, and of Pawlowsky, in 1869, assume to have 

 confirmed their observations, and to have demonstrated that the nerves decus- 

 sate completely at the commissure and are each distributed exclusively to the 

 eye of the opposite side. Purely anatomical researches upon such a delicate 

 point as this are uncertain ; and it is with the aid of pathological observa- 

 tions, that positive conclusions have been reached. With the views advanced 

 by Mandelstamm and by Michel, it is perhaps possible to explain cases of 

 hemiopsia due to tumors situated at some parts of the chiasm, but not heini- 

 opsia following deep-seated injury or disease of the brain near the origin of one 

 of the optic tracts. Two such cases we shall quote from Drs. Keen and Thom- 

 son (see page 41), and these are sufficient to lead us to doubt the accuracy of 

 the view of complete decussation, at least until it shall have received full con- 

 firmation. The new view, however, is fully discussed in the following articles, 

 to which the reader is referred : 



MANDELSTAMM, Ueber Sehnervenkreuzung und Hemiopie. Archiv fur Ophthal- 

 mologie, Berlin, 1873, Bd. xix., S. 39, et seq. 



MICHEL, Ueber den Ban des Chiasma nervorum opticorum. Ibid., S. 59, 

 et seq. 



