THE PYRAMIDAL TRACT 515 



Rat 



The cortico-spinal tract of the albmo-rat is situated in the 

 posterior funiculus in the form of two compact bundles separated 

 from each other by the posterior median septum and bounded 

 anteriorly and laterally by the gray substance of the cord, while 

 posteriorly it extends to about one-fourth of the thickness of 

 the posterior funiculus (Ziehen '99, Bechterew '90, Goldstein '04, 

 Van der Vlort '06, and Ranson '13). 



As seen in figure 1, from pjTidine-silver preparations, the 

 tract is quite clearly outlined; its fibers not intermingling, to 

 any extent, with others in the posterior funiculus. The axons 

 are closely packed and very numerous. Most of them are very 

 small and these are stained dark brown or black. The larger 

 axons are stained yellow while the myelin sheaths are colorless 

 and the neurogha is stained very faintly or not at all. The 

 preponderance of the very small axons gives the tract a char- 

 acteristic brown color. 



The Pal-Weigert preparations showed the tract as a light 

 grayish-blue area in the posterior funiculus. It contains numer- 

 ous very fine medullated fibers and a few of medium size. The 

 myelin sheaths are not equally stained and some are very promi- 

 nent while others are barely visible. They are not as closely 

 packed as the axons in the pyridine-silver preparations. The 

 majority of the axons are non-medullated. 



The medullated fibers vary markedly as to their size (diameter). 

 The largest ones as found in these sections, measured 8.44 mic- 

 rons, while the majority of these fibers ranged from 1 micron to 

 3.2 microns. 



Guinea-pig 



The pyi^amidal tract of the guinea-pig is also found to be 

 situated in the posterior funiculus. It differs from that found 

 in the rat in that it is not a compact uniform mass but the two 

 tracts have the form, more or less, of the letter 'V and the 

 posterior boundary is indefinite. It occupies the most anterior 

 part of this funiculus and its 'horns' occupy fully two-thirds 

 of the thickness of the posterior funiculus. The posterior septum 

 separates the tracts. 



