Allen, Association in the Guinea Pig. 345 



before they can effect an entrance. A further cause of darken- 

 ing in the ground bundles around the margin of the gray sub- 

 stance may be that the longitudinal bundles are there more 

 dense. In other words, in the section there may be more trans- 

 versely cut fibers in the ground bundles than in the peripheral 

 white substance. An enumeration of the fibers shows that in 

 a given area tliere are in the lateral funiculus 16.7% more cross 

 sections of fibers close to the ventral column than at the peri- 

 phery. In the ventral funiculus the difference is 20.2% in 

 favor of the given area close to the ventral column as opposed 

 to a peripheral area in the same funiculus. 



Thoracic Level- 



At the thoracic level the lightly meduUated area in the 

 fasciculus cuneatus at the postero lateral margin of the cervix 

 columnae dorsalis (the pyramidal tract) is less well medullated 

 than in the cervical region. It extends further toward the me- 

 dian line, so that in its ventral half it approaches nearer the 

 median septum and extends ventrally as far as the commissura 

 posterior. 



The light area ventral to the lateral tip of the substantia 

 gelatinosa appears in the thoracic segments. The fasciculus 

 gracilis shows the two subdivisions commented upon in the de- 

 scription of the cervical level, the dorso-medial being the less 

 well medullated. 



In the thoracic as in the cervical cord the number of me- 

 dullated fibers in the gray substance is worthy of attention. 



Lumbar Level- 



The light area in the funiculus cuneatus of the lumbar 

 level occupies a position corresponding to that noticed in the 

 section of the cervical, i e. , an oval area following the border 

 of the cervix of the dorsal column, from the edge of the sub- 

 stantia gelatinosa to the commissura posterior, but separated 

 from the latter by a well marked area of medullated fibers. 

 This area is somewhat smaller than the corresponding area in 

 the cervical, and contains a larger number of scattered medul- 

 lated fibers than appear at the levels above it. 



[53] 



