Hardest Y, Medulla Spinalis of the Elephant. 145 



crease again in thoracic XII, and in the intumescentia lumbaUs, 

 attains dimensions nearly equal to those in the cervical segments, 

 and then rapidly decreases into the conus medullaris. These 

 variations are little more than an expression of the variations 

 in the lateral diameter of the specimen. The increased fasci- 

 culi proprii attendant upon the greater amount of substantia 

 grisea in the lumbar segments account almost wholly for the 

 increase of substantia alba in the intumescentia lumbalis, for 

 naturally the axones of longer course must be less abundant 

 here than in the thoracic segments, though they may not be so 

 compactly bundled. However, that the funiculi posteriores 

 (measurements 9 and 10) have a greater width in the lumbar 

 than in the mid-thoracic segments may be explained as not only 

 due to the increased fasciculi proprii dorsales, but to some ex- 

 tent as due to the presence of a greater number of the shorter 

 descending divisions of the entering afferent axones attendaat 

 upon the larger lumbar radices posteriores, and also to the con- 

 sequent greater number of collateral branches. 



That the funiculi posteriores are wider in the cervical than 

 in the lumbar segments must be almost entirely due to the nec- 

 essarily greater bulk of cerebro-spinal axones ascending in 

 them, for the fasciculi proprii attendant upon the abundance of 

 substantia grisea and the descending branches of afferent axones 

 and their collaterals can hardly contribute more to the increase 

 of these funiculi than they do in the lumbar segments. 



It is interesting to note that the space attributed to the 

 funiculi laterales (measurement 8) does not vary so much at the 

 different levels as would be expected from our knowledge of the 

 components of these funiculi in the medulla spinalis of man 

 and certain other mammals. The absence of a more graded 

 decrease may be explained to some extent by the, at least, 

 partial absence from these funiculi of the fasciculus cerebro- 

 spinalis lateralis (crossed pyramidal tract). An attempt will be 

 made to show that this fasciculus runs instead in the commis- 

 sura grisea. A glance at figures 7, 14, 10, and ii {F. c. i.) 

 will convince one that the fasciculus occuring here decreases 

 gradually in passing through the cervical and thoracic segments. 



