230 Jounial of Comparative Neurology and Psychology. 



ment if we bear in mind the relati\"ely high sensibiHt)- of the 

 skin of the neck, and the small motor supph' to the muscles. 

 But that the roots going to the arm should show a relatively 

 smaller number of sensory fibers than does the leg, needs some 



TABLE VII. 

 Showing the ratio between the number of nerve fibers in the ventral 

 and the dorsal roots of man at different levels in the cord. 



203,700 653,627 1:3-2 



explanation, as the arm has a greater number of sensory fibers 

 per square unit of surface than the leg. The reason for this 

 smaller ratio for the arm is due to the fact tnat the arm has a 

 much larger number of motor fibers in proportion to its weight 

 than has the leg, and this fact accounts for the ratio found. 



^. Comparison of the Ratio of the Number of the Ventral 

 and Dorsal Roots of the Frog, the Rat and Man. — Having now 

 the ratio between the number of fibers in the ventral and the 

 dorsal roots of man, it may be of interest to compare this ratio 

 with similar ratios for some lower animals. We have found the 

 ratio between the number of fibers in the ventral and dorsal 

 roots of man to be 1:3.2. This ratio can also be obtained by 

 taking three typical roots only. 



\"entral Dorsal Ratio 



C. VI 11,794 46,549 



Th. IV 7.625 11,375 



L. II 6.014 23,640 



25,433 80,564 1:3.2 



This being so, we have some ground for believing that the 

 ratio between these three pairs of roots in other animals may 

 also give ver)' closely the ratio between all the fibers in the ven- 

 tral and dorsal roots. 



Hatai (1903) counted in the white rat the nerve fibers in 

 the ventral and the dorsal roots of three pairs of the left spinal 

 nerves (C. VI, Th. IV and L. II), and obtained the ratio 

 1:2.3. 



