150 TEIJI HOSHINO 



separation of the cerebellum and brain stem in frontal sections, 

 in the dorsolateral part of the brain stem, a thick fiber group 

 runs up to the median line. Laterally, there is seen, outside of 

 the brain stem, the peripheral trochlear nerve which is cut paral- 

 lel to the fibers. Proximally to the trochlear nucleus, we see a 

 massive fiber bundle which forms the ventrolateral part of the 

 brain stem to the optic lobe. 



The breadth of the cerebellar peduncle in the affected birds 

 at different heights shows a marked reduction in its distal and 

 middle region, with only a slight reduction, however, in its proxi- 

 mal region. If we measure the cerebellar peduncle at its narrow- 

 est place, namely, from the lateral edge of the cerebellar ventricle 

 to the place where the external lateral surface bends and turns 

 medially, we obtain the following measurements: At the distal 

 region of the cerebellar peduncle, its breadth measures 1.837 

 to 2.004 mm. in the normal and 1.253 to 1.336 mm. in the 

 affected specimen; in the middle portion of the peduncle, 1.921 to 

 1.754 mm. in the normal and 1.320 to 1.253 mm. in the affected, 

 and in the proximal portion, 1.169 mm. in the normal and 1.002 

 mm. in the affected pigeon. Moreover, this reduction is chiefly 

 seen in the lateral fiber bundle of the cerebellar peduncle, espe- 

 cially in the distal and middle portions of the peduncle. 



In the cerebellar peduncle there are many other fiber bundles 

 besides those already described. These were studied by Frenkel, 

 Y\ allenberg, and Shimazono and will be briefly discussed. 



The tractus octavo-cerebellaris goes up directly from the acous- 

 tic area, passing through the cerebellar peduncle to the lateral 

 region of the cerebellar body and enters the cortex of the cere- 

 bellum. The tractus octavo-floccularis runs from the large cell 

 nucleus and nucleus parvocellularis to the medial part of the 

 peduncle, forming an arc obliquely to the peduncle, and enters 

 the lobus lateralis. 



The fibers in the tractus octavo-floccularis in the affected sec- 

 tion are separated into a few small fiber groups, while in the nor- 

 mal they run as one solid bundle in the ventral part of the pe- 

 dunclejasa result of this, the fibers in the affected specimen are 

 reduced in volume without showing any kind of distinct degener- 



