BRAINS AND SPINAL CORDS IN ATAXIC PIGEONS 151 



ativc processes in their myelin sheaths. The nucleus parvo- 

 cellularis, as already indicated, does not stem to have quite the 

 same semilunar shape as the one in the normal section and the 

 cells appear to be a little smaller, even though tie' difference is 

 not striking. 



The tractus octavo-cerebellaris is seen besl a little proximal 

 to the above section of the tractus octavo-floccularis. Here the 

 tract runs from the acoustic area dorsally, crossing the tractus 

 octavo-floccularis, and then passes through the ventral group of 

 the lateral cerebellar nucl< i in separated small bundl< e to reach the 

 dorsal part of the cerebellar body. This bundle diverges into 

 ten to twelve fiber groups in both the affected and normal sec- 

 tions; the whole breadth of the bundle in the affected pigeon 

 measures 0.114 mm., while in the normal it is 0.151 mm. No 

 degenerative processes, however, could be seen. 



The tractus quinto-cerebellaris emerges from the sensory tri- 

 geminal nucleus, passes the cerebellar peduncle medial to the 

 spinocerebellar tract to the cerebellar body. This tract is well 

 seen in both birds and there is no recognizable difference between 

 them. The sensory nucleus of the trigeminal nerve in the 

 affected birds is in good condition, as already observed. 



The tractus bulbo-cerebellaris runs from the nucleus lateralis, 

 located ventral to the facial nerve stem in the peripheral border 

 of the medulla, to the lateral side of Deiter's nucleus dorsalward 

 to disappear in the tractus spino-cerebellaris in the cerebellum. 

 This tract is not easily differentiated in either our normal or 

 affected sections, but it must be included in the reduced lateral 

 fiber groups of the medulla or the cerebellar body. The tractus 

 cerebello-spinalis, an efferent tract, which should be in the cere- 

 bellar peduncle, is difficult to define, but so far as the cerebellar 

 nucleus (medialis) is concerned, which is supposed to be the 

 nucleus of origin, there is no change in it whatever. 



The tractus cerebello-mesencephalicus enters the brain stem 

 at the ventral region of the cerebellar peduncle and runs median- 

 ward. Wallenberg and Shimazono proved this by injury of the 

 nucleus cerebellaris lateralis. In our sections this tract, which 

 is the brachium conjunctivum, appears medial to the sensory 



