BRAINS AND SPINAL CORDS IN ATAXIC PIGEONS 153 



0.367 mm. in the normal and 0.334 mm. in the affected, 0.468 

 mm. in the normal dorsoventrally. The nucleus ruber with the 

 decussation of the brachium conjunctivum, are both quite normal 

 in size and in number of cells. The cells in the red nucleus meas- 

 ure on the average 28.5 n in both the normal and affected sections. 

 In the nucleus isthmi in the brain stem and nucleus mesenceph- 

 alicus lateralis in the optic lobe, no distinct difference from the 

 normal can be detected. 



SUMMARY AND DISCUSSION 



We may summarize here the chief differences observed in the 

 affected birds. Macroscopically, the central nervous system 

 especially, cerebellum, medulla oblongata, and spinal cord are 

 reduced in size in all the affected birds. A reduction in weight, 

 chiefly in the distal portion of the brain, is present both absolutely 

 and relatively in reference to the body weight and the weight of 

 the whole brain in comparison with those of the normal. 



In the cerebellum, the molecular layer is decidedly reduced in 

 its thickness, showing microgyri throughout. Though it is es- 

 tablished that the direct spinocerebellar tract distributes its 

 terminals in the ventral lobuli of the anterior lobe (Ingvar, '18), 

 no localization in regard to the reduction in the cortex of the cere- 

 bellum in our cases could be seen. The reduction of the cortex 

 in the lateral part of the cerebellar body, however, is relatively 

 striking and also is constant in all affected birds. The Purkinje 

 cells, even though they exhibit sometimes a little more slen- 

 der shape and shorter dendrites, show no decided regressive alter- 

 ations. Neither is there seen a total defect or any increase in 

 the nerve elements in the cortical layer. The medullated layer 

 is paler by the Weigert, owing to the thin myelin sheaths of each 

 fiber, but not from swelling or segmentation. The large nuclei 

 in the cerebellum, the nucleus cerebellaris medialis and lateralis, 

 as well as the tractus cerebello-mesencephalicus, are in as good 

 condition as in the normal. The pedunculi cerebelli show strik- 

 ing reduction, sometimes to one-half the normal. Thus is well 

 seen in the inferior and middle portion of the peduncle, but not so 



