22)5 



The axis cylinder generally stains deeply and uniformly throughout 

 its whole length with either Heidenhain's, Kenyon's or Sahli's me- 

 thod. Frequently, however, the proximal part of the fibre immediately 

 adjacent to the cell takes the stain but slightly, the protoplasm of that 

 part of the neurite staining precisely like that of the cell proper, with 

 which it seems to be continuous and identical (Fig. 8, c). The axis 

 cylinders are throughout their course non-medullated, but Schwann's 

 sheath is present showing the characteristic nuclei. 



The neuntes of these giant cells form a pair of conspicuous and 

 characteristic fibre bundles lying symmetrically, one on either side of 

 the cord lateral and dorsal to the canalis centralis (Fig. 3, d). The 

 fibres may be distinguished throughout their course from other ad- 

 jacent fibres by three characteristics: — 1) the absence of a medullary 

 sheath; 2) their large size; 3) their aggregation into a characteristic 

 bundle. Each bundle in the region of the medulla consists of from 

 nine to twelve fibres. At the posterior limit of the series of giant 

 cells the bundles in the chord consist of four or five fibres only. The 

 number increases cephalad of this point as additional neuntes enter 

 the bundles. 



In their course through the cord the fibres lie within the dorsal 

 horn of the gray substance close to its ventral limit. Here they are 

 loosely aggregated, and have a somewhat undulating course; but in 

 the medulla they are closely pressed together, so that in cross section 

 each fibre has a more or less sharply polygonal outline. In the me- 

 dulla the bundles rise dorsally to the level of the floor of the fourth 

 ventricle, and at the same time curve laterad. Forward of this they 

 again become more ventral. They are often abruptly deflected from a 

 direct path in order to pass around the deep roots of the cranial 

 nerves. In the region of the fifth cranial nerve the fibre bundles curve 

 laterad and ventrad and finally pass out through the ventral root of 

 the trigeminus nerve. The fibres have been traced out into the fifth 

 nerve, and have been followed through their course in a considerable 

 number of series cut in the frontal, sagittal, and transverse plane.-, 

 but the course and ending of the fibre bundles posteriorly yet remains 

 to be worked out. 



It is difficult to follow that branch of the neurite which does not 

 enter the lateral bundle, owing in part to the presence of the peculiar 

 filiform neuroglia structures in that part of the cord, which so strongly 

 resemble the non-medullated fibres of the giant ganglion cells as to 

 render the identification of the latter very difficult. The evidence 



