354 C. JUDSON HERRICK 



THE SENSORY ROOTS OF THE CRANIAL NERVES 



The sensory root fibers which enter the medulla oblongata of 

 Amblystoma in general bifurcate into ascending and descending 

 branches immediately upon entering the stratum album, these 

 ascending and descending root fibers being arranged in separate 

 fascicles related with their respective roots. These fascicles have 

 a definite arrangement which is maintained without essential 

 modification throughout most of the length of the oblongata, the 

 arrangement being such that the less highly specialized and 

 presumably more primitive systems lie farther ventrally than the 

 more highly differentiated systems (figs. 3 and 7 to 16). 



The only important deviation from this rule is presented by the 

 visceral sensory fascicle of root fibers, viz., the fasciculus soli- 

 tarius (fasciculus communis of Osborn). Comparative anatomy 

 and comparative embryology show that the fasciculus solitarius 

 and its nucleus primitively lie ventrally of the spinal V root and 

 its nucleus, an arrangement which has been secondarily modified 

 in urodeles by a ventral movement of spinal V and VIII root fibers 

 so that they come to lie superficially and ventrally of the visceral 

 sensory root fibers and primary centers. The secondary visceral 

 tract here, as in fishes, retains its primitive position below the 

 spinal V root (figs. 7 to 16, tr.v.a.). 



Within the sensory lamina of the oblongata the ascending and 

 descending fibers of the chief sensory V root form the most ventral 

 fascicle of the series. Immediately dorsally of these lie the two 

 fascicles of VIII root fibers. The visceral sensory fibers of the 

 VII and IX nerves pierce the fascicles of VIII fibers to enter the 

 brain. The most rostral visceral sensory rootlets of the X nerve 

 also pass through the VIII root fascicles, but the more caudal 

 vagal rootlets of this system enter farther ventrally through the 

 spinal V root. All visceral sensory fibers enter the fasciculus 

 solitarius and here some (perhaps all) of them divide into ascend- 

 ing and descending branches. Unlike the other fascicles of root 

 fibers, the fasciculus solitarius for its entire length lies deeply 

 embedded within the substance of the oblongata, between the 

 stratum griseum and the VIII roots. Dorsally of the fasciculus 

 solitarius and VIII roots is a longitudinal tract of correlation 



