4 Journal of Comparative Neurology. 



tectum has gone on and perhaps nearly reached its Hmit. The 

 left tectum is fully four times the size of the right but the inti- 

 mate structure is not obviously different. 



Passing to the oculo-motor apparatus, the third nerve roots 

 are both present and it requires close examination to discover 

 anything unusual in them but the fibres are atrophied to some 

 extent. The niduli, on the other hand, are nearly completely 

 atrophied. The cells are nearly all shrunken and shrivelled and 

 lie in large perilymph spaces. The contrast between these cells 

 and the unaltered cells of the interpeduncular niduli is instruc- 

 tive. (See Fig. lo.) In this case the fourth nerve has shared 

 the fate of the third. While the atrophy has been more marked 

 on the right side than on the left both are affected. The series 

 unfortunately does not extend to include the sixth, but it is 

 probable that the same condition existed in it. Our material is 

 inadequate to illustrate the changes in the tracts and niduli of 

 the second order. 



In attempting to analyse the various alterations with which 

 we here have to do it is necessary to distinguish degeneration from 

 atrophy and aplasy, the latter being a failure to develop as a 

 result of operative interference. See Singer and Munzer, 

 Denksch. Kaiserl. Akad. z. Wien. 1890. 



DESCRIPTION OF FIGURES. 



Fig. I. Transection of rabbit brain (No. 292) immediately cephalad of 

 the optic tecta, showing the atrophy of the tracts of the left side. The section 

 is not exactly transverse. 



Fig. 2. Section through the tecta showing the atrophy of the left side and 

 in the nidulus numbered "3." 



Fig. 3. Section at the roots of the third nerves showing the absence of the 

 right nidulus and root. 



Fig. 4. Portion of a section through the fourth nidulus showing that it is 

 not affected by the atrophy. 



Fig. J". Section at the exit of the sixth showing the absence of the fibres 

 of the right side. 



Fig. 6. Part of a section further caudad showing the complete atrophy of 

 the right nidulus. 



Fig. 7. Part of a transection of a rabbit brain (No. 297) from which the 

 left eye was removed at birth, showing the degeneration of the right tract. 



Fig. 8. A section further caudad. 



Fig. g. Section at the habenre showing atrophy in tract of second order 

 on the right as well as in the optic tract. 



Fig, 10. Cells from the nidulus of the third (A) and from the interpedun- 

 cular nidulus (B) to demonstrate the atrophic changes in the former. 



Fig. II. Normal and degenerate portions of optic tract from 292, 8. 



