PRINCIPAL REGIONS OF SPINAL CORD 87 



out the series. The average thickness is greater in the thoracic 

 region than in either of the other two regions, being 0.84 mm. 

 in the thoracic, 0.61 mm. for the cervical and 0.54 mm. for the 

 lumbar region. It is noticeably thick in the thoracic regions 

 of the ox, bear, hog, sheep, kangaroo, fox, Java monkey, and 

 rabbit. 



Reticular formation. There is considerable variation in the 

 amount of reticular formation as shown in the figures. This 

 reticular network is believed to be formed, at least in part, by 

 a dispersion of the lateral portions of the grey figure (1) by 

 bundles of longitudinally coursing association fibers (fasciculi 

 proprii), (2) by fibers passing out of the grey figure into the 

 white substance and (3) by the fibers from the crossed pyramidal 

 tracts leaving the lateral funiculus and entering the grey figure 

 to terminate about ventral horn cells. However, the lateral 

 pyramidal tract may have httle to do with it for the animals 

 which have their pyramidal tract in the dorsal funiculus have the 

 reticular formation as well, often better, marked than do animals 

 which have the pjTamidal tract in the lateral funiculus. 



Nucleus dorsalis. A nucleus dorsahs (Clarke's column) is well 

 marked in the thoracic region of by far the greater number of 

 animals here studied. In the following six mammals, kangaroo, 

 opossum, agouti, guinea-pig, rat, and mouse the nucleus is not 

 clearly defined. As will be mentioned, the pyramidal tract in 

 each of these animals courses, probably in the dorsal funiculus 

 and it may be that the dorsal position of this tract is in some 

 way associated with the modified appearance of the nucleus 

 dorsahs. These observations are based on the study of Weigert 

 sections in which the cell-bodies are not stained, but the position 

 and usually a very good outhne of the cell-body may be seen. 



Proportion of grey substance to white. An idea of the relative 

 amount of grey substance as compared with white in the spinal 

 cord is best obtained from a study of the ratios of the absolute 

 areas of the two. Table 7 gives such a ratio for the three reg- 

 ions of the cord of each animal. The area of the grey figure is 

 to the total area of the white substance as 1 is to the figure 

 given in the table. All of the spinal cords, with the exception of 



