28 Journal of Comparative Neurology and Psychology. 



always be seen to protrude along the dorral branches instead of 

 upon the trunk. 



For the entire 21 nerves included here the general average 

 ratio of meduUated fibers in the ventral roots to those in the 

 dorsal roots is 1:1.8. Birge's work with Rana csculenta (^'62) 

 gives the similar ratios of 1:1.7 and 1:1.8, and the author's pre- 

 vious work gives for the same three nerves an average ratio of 

 1:1.6, and for the Vlth nerve alone of 19 specimens, a ratio of 

 1:1.8. This similarity in the proportion of fibers in the motor 

 and sensory roots indicates that whatever other changes may 

 take place, a more or less fixed numerical relation is maintained 

 becween the two. 



For all of the spinal nerves of one side of the frog, Birge's 

 figures give a ratio of ventral to dorsal root fibers of 1:1.2. Hatai 

 ('02), investigating three of the spinal nerves of the white rat, 

 finds in every case more fibers in the dorsal than in the ventral 

 root. The excesses varied from 2 to 2.9 times the ventral root fi- 

 bers and gave an average ratio of 1:2. 5. Ingbert ('04) has recently 

 completed the enumeration of the fibers in the dorsal and ven- 

 tral roots of all the spinal nerves of one side of an adult man 

 and it is interesting to note that the ratio found by him is con- 

 siderably higher than that found in either the frog or the rat. 

 He finds in the sums of all the roots of one side that the 

 ventral root fibers are exceeded by those in the dorsal roots in 

 the ratio of 1:3.2. In comparing the different regions of the 

 spinal cord, he further shows that the excess of dorsal root 

 fibers runs appreciably higher in most of the cervical and lumbo- 

 sacral nerves than in any of the thoracic region. Hatm's fig- 

 ures indicate this to be true for the white rat also, and to some 

 extent the figures available show it true for the frog. It is ex- 

 plained as entailed in the requirements for the inervation of the 

 relatively greater skin area of the upper and lower limbs. 



Hatai, dealing with rats of different ages, found further 

 that the excess of dorsal root fibers was greater in the young 

 than in the adult, and thus with reason claims that the ventral 

 root fibers increase in number more rapidly between the periods 

 of medullation and maturity than do the dorsal root fibers. If 



