158 Journal of Comparative Neurology, 



motor tracts, come into intimate association with cells which in 

 form and relations seem to be homologous with the niger cells 

 (i. e. , cells in hypoaria). The cerebellar tract also enters into 

 relation with this nidulus. Following this clue we trace the 

 same relation in all the higher groups. It seems safe to affirm 

 that the site of motor association with the cerebellum is in this 

 cell-clustre. It need not surprise us that in a disease which is 

 due primarily to a disturbance of the cortex and notably the 

 motor-cortex, the projection centres of the centrifugal tracts 

 should be especially injured. It seems to amount to an inci- 

 dental confirmation of our anatomical observation and physio- 

 logical theory that the nidulus niger is the only region of the 

 mesencephalon in which pronounced indications of degeneration 

 are observed. The nidulus extends from the exit of the third 

 nerve to the point where the peduncles enter the striatum and 

 throughout its entire extent nearly every cell is more or less 

 completely degenerate. In some cases the whole cell body is 

 reduced to a vague patch of yellow granules, while in others 

 bands or stripes of unaltered protoplasm remain. Frequently 

 the incipient stages of decay are seen in a deep pigmentation 

 near the nucleus or one process only is disorganized. The char- 

 acteristic features of these degenerate cells are best gathered from 

 the figures. 



Plate D, Fig. j, illustrates a niger cell far cephalad near y 

 in Fig. I. It well shows the two axis-cylinders of a switch cell. 

 Fig. 2, is a portion of the same field with cells in various stages 

 of degeneration. One cell especially illustrates the rare condi- 

 tion where a bulbous swelling occurs upon a process. In some 

 cases the nucleus itself has suffered partial destruction. Fig. 4., 

 contains two unaltered cells giving a good idea of the various 

 processes though the finer ramifications of the protoplasmic pro- 

 cesses are not bought out. Figs, j and ^, of Plate C, illus- 

 trate the changes seen in the same nidulus at the level of the 

 third nidulus. 



Figs. I are 2 and portions of transverse sections at the 

 level of the pulvinar and third nidulus respectively, for orienta- 



