Sargent, Giant Gaitglion Cells of Ctenolabnis. 193 



unmedullated, but Schwann's sheath is present showing the char- 

 acteristic nuclei. 



The neurites form two distinct and characteristic fiber 

 bundles lying symmetrically on either side of the cord lateral 

 and dorsal to the canalis centralis (Fig. 3, /. /. b.^ They may 

 be distinguished throughout their course from other adjacent 

 fibers by three characteristics, (i) the absence of a medullary 

 sheath ; (2) their large size ; (3) their aggregation into a char- 

 acteristic bundle. Each bundle in the region of the medulla 

 consists of from nine to twelve fibers. At the posterior limit of 

 the series of giant cells in the cord the bundles consist of four 

 or five fibers. The number increases cephalad of this point as 

 the neurites enter the bundles. 



In their course through the cord the fibers lie within 

 the dorsal horn of the grey substance close to its lower limit. 

 The fibers here are loosely aggregated having a somewhat un- 

 dulating course. In the medulla the bundles rise to the level 

 of the floor of the fourth ventricle, and at the same time curve 

 laterad. Forward of this they again become depressed. In the 

 medulla the fibers are closely pressed together, so that in cross 

 section each fiber has a more or less sharply polygonal outline. 

 In the medulla the bundles are often abruptly deviated from 

 their direct path in passing around the deep roots of the cranial 

 nerves. In the region of the fifth cranial nerve the fiber bun- 

 dles curve laterad and ventrad and pass out through the ventral 

 root of this nerve. The fibers have been traced out into the 

 nerve, and have been traced through their course in a consid- 

 erable number of series cut in the frontal, sagittal and trans- 

 verse planes. 



The course and ending of the fiber bundles posteriorly yet 

 remains to be worked out. That branch of the neurite which 

 does not enter the lateral bundle is difficult to follow, owing in 

 part to the peculiar filiform neuroglia structures in that part of 

 the cord, which strongly resemble the non-medullated fiber of 

 the giant ganglion cells. The evidence derived from the study 

 of many preparations indicates that this branch turns laterad 



