390 DAVIDSON BLACK 



The relations of the motor V nucleus and its emergent root 

 in R. esculenta and R. fusca are essentially similar to those ob- 

 taining in R. catesbeana. In Bufo, however, the rostro-caudal 

 extent of the motor V nucleus is somewhat greater than in the 

 Ranidae examined, though its position relative to the motor 

 VII nucleus remains practically unchanged (fig. 9) . 



In urodeles the motor V nucleus is apparently not so large 

 nor so sharply delimited from the motor tegmental elements as 

 in anurans. The relation of the nucleus to the exit level of its 

 root is subject to some variation among urodeles (v. fig. 10). 

 In all cases, however, in both urodeles and anurans, the motor 

 V nucleus is situated dorsally and lies in the periphery of the 

 periependymal gray beneath the poorly defined sulcus limitans. 



Nerves III and IV 



In R. catesbeana the trochlear nucleus consists of a well 

 marked and definitely circumscribed collection of multipolar 

 perikaryons which lie beneath the periependymal Sylvian gray 

 in a trough-like excavation of the fasciculus longitudinalis 

 medialis, on a level rostrad of the ganglion interpedunculare. 

 The trochlear nucleus lies immediately caudad of the oculo- 

 motor nucleus, from which it is separated however by a small 

 but distinct space. In the specimen from which the recon- 

 struction chart figure 9 A was prepared, the trochlear nucleus 

 was slightly more than half the length of the oculomotor nucleus, 

 and the motor perikaryons which compose both nuclei were of 

 similar size and morphology. 



The fibers of the trochlear nerve are collected on the lateral 

 aspect of the nucleus and pass first laterad and then dorso- 

 caudad to reach the superior medullary velum. In the latter 

 structure the trochlear decussation occurs and the nerves then 

 emerge on either side of the mid-dorsal line (v. figs. 6 and 7). 

 It is possible that some of the fibers entering the trochlear 

 nerves at their emergence are uncrossed and are derived, as 

 McKibben (43) has suggested in Necturus, from the mesen- 

 cephalic nucleus of the trigeminal nerve. 



