Secondary Degeneration following Unilateral Lesions etc. 315 



anterior and mesial to tlie pyramidal i)uii(11es. 'I'liis fine degeneration 

 stops abruptly at the middle line, none is visible on the opposite side 

 (see fig. 2. pi. XVI). It is present in every case, and often exceedingly 

 abundant. (Fine degeneration is usually interpreted as terminal degener- 

 ation, i. e. the degeneration of collateral or terminal fibres.) No fibres 

 are seen to pass backwards from the pontine pyramidal bundles to- 

 wards the grey matter of the floor of the 4*^ ventricle as had been 

 observed from the crusta in the mid-brain, and no fine degeneration 

 is present in that grey matter. 



In the lower levels of the pons after the pontine bundles have 

 re-united to form the anterior pyramid (in the cat the anterior pyra- 

 mids are formed and come to the surface in the lower part of the 

 pons) a few fibres begin to leave the posterior aspect of the de- 

 generated pyramid. Most of these cross the median raphe and are 

 lost amongst the internal arcuate fibres of the formatio reticularis of 

 the opposite (right) side, while some disappear in that of the same 

 side. Similar fibres continue to pass backwards from the degenerated 

 pyramid throughout its whole extent in the bulb until the upper ex- 

 tremity of the true pyramidal decussation is reached. They run, not 

 towards the grey matter in the floor of the 4**^ ventricle, or around 

 the central canal in the lower or closed part of the medulla, but more 

 lateralwards towards the base of the substantia gelatinosa of Eolando, 

 and they cannot be traced far into the formatio reticularis. No fine de- 

 generation is seen in the grey matter of the floor of the 4*^ ventricle, 

 in any part of the medulla oblongata, although this was carefully 

 looked for, more especially in the region of the hypoglossal nucleus. 

 Some large pigmented cells, in one or two cases, are present in the 

 hypoglossal nucleus on each side, but this pigmentation, which is not 

 uncommon in the large motor cells of the cat, can easily be distin- 

 guished from degeneration by examination with the high power. In 

 many of the cases a few degenerated fibres can be seen, cut trans- 

 versely, situated in the formatio reticularis, anterior and internal to 

 the base of the substantia gelatinosa of Eolando on the right (oppo- 

 site) side. These occupy in the upper part of the medulla relatively 

 the position of the crossed (heterolateral) pyramidal fibres in the lower 



