THE PYRAMIDAL TRACT 513 



to many fine and medium-sized fibers which have a medullary 

 sheath, there are a goodly number of non-meduUated fibers. 

 From this he concluded that in the case of the rat, the medullation 

 of the fasciculus cortico-spinalis was incomplete. Scattered 

 sparsely throughout the entire white substance of the rat's cord 

 were seen non-meduUated fibers but those in the pyramidal 

 tract were very numerous and compact. There is a great differ- 

 ence in the size of the medullated fibers of this tract in different 

 animals. In the rat the small size of the fibers seems to be associ- 

 ated with the fact that large numbers are non-medullated. Do 

 the proportion of medullated fibers and the size which the largest 

 medullated fibers have attained, go hand-in-hand as indices 

 of the stage of development that the tract has reached? With 

 this problem in mind it seemed of interest to determine the size 

 of the largest of these fibers in the pyramidal tract and the 

 relationship between the medullated and the non-medullated 

 fibers in this tract in different mammals, 



TECHNIQUE 



The arbitrary level of the seventh cervical segment was taken 

 and all sections were made at this region. In most cases, more 

 than two animals of each species were used, since poor preparations 

 were rejected. For each species cords were selected which were 

 of about the same size. Some of the material was stained ac- 

 cording to the pyridine-silver method (Ranson '12), the steps 

 of which are as follows: 



The tissue is denuded of its dural sheath and placed into 



(1) Absolute alcohol plus 1 per cent strong ammonia for forty-eight hours 



(2) Distilled water, wash for two minutes 



(3) Pyridine, for twenty-four hours 



(4) Distilled water, for twenty-four hours with many changes of the water 



(5) Silver nitrate solution 2 per cent (2%), for three days, kept in the dark and 

 at 35°C. 



(6) Distilled water, rinsed 



(7) Pyrogallic acid 4 per cent (4%) in 5 per cent (5%) formalin, for one or two 

 days 



(8) Dehydrate in alcohol 



(9) Clear in xylol 



(10) Imbed in paraffin 



(11) Cut sections 10 microns thick, mount and examine 



