Johnston, TJic Bi-ahi of Petromyzo7i. 5 



roots of the spinal nerves, is entirely free from these thick fibers. 

 This area may be divided into two portions which are quite dis- 

 tinct in the region of the first three spinal nerves : a lateral 

 portion which is made up of somewhat coarser and more deeply 

 staining fibers, and a median portion whose fibers are very uni- 

 form in diameter, finer than those of the lateral portion, and 

 take a pale stain in iron haematoxylin. The lateral, deeply 

 staining area receives the fibers of the dorsal roots, the median 

 pale portion seems to be continuous cephalad with the fasciculus 

 communis center only. 



The transition from the cord to the medulla is marked by 

 the following changes (Figs. 3—7). The cord becomes com- 

 pressed or contracts laterally until it first assumes a round form 

 and then becomes thicker dorso-ventrally than from side to 

 side. At the same time the central canal becomes larger and 

 oval in outline, rises slightly toward the dorsal surface, then 

 becomes wider near the dorsal surface and finally opens out into 

 the wide ventricle with choroid roof. As the cord assumes the 

 cylindrical form the large cells in the lateral part of the grey 

 gradually approach the ventral raphe until they take up. the 

 characteristic position of ventral horns and by this movement 

 the median group of Mullerian fibers are crowded into a com- 

 pact bundle between the ventral horns and the raphe. At the 

 same time that portion of the grey which lies, lateral to the 

 canal in the cord becomes greatly increased and thickened 

 dorso-ventrally, the dorsal or giant cells disappear, and the 

 thickened grey mass extends up at the sides of the enlarged 

 canal and becomes continuous with the nucleus of the dorsal 

 funiculi. This latter appers in line with the lateral, deeply stain- 

 ing portion of the dorsal white matter described above. The 

 difference in the staining properties of these two areas seems 

 to disappear as the medulla is approached but the median area 

 remains free from cells except in the immediate neighborhood 

 of the central canal, and just before the canal opens out into 

 the fourth ventricle the median areas of the two sides are con- 

 nected by a commissure over the ventricle. This is the com- 

 missura. infima Halleri, and marks the tracts in question as 



