MOTOR NUCLEI IN PHYLOGENY 75 



a dorsal position just laterad of the fasciculus longitudinalis 

 medialis. Both cell groups in this form were situated on a 

 level caudad of the plane of exit of the motor V root (Willard, 43, 

 p. 48, and Kappers, 31, p. 40). 



In general it would appear from the material examined that 

 the synapsid (Damonia and Chelone) and diapsid (Alligator) 

 forms are characterized by the dorsal situation of most of the 

 motor V perikaryons, while in the Squamata (Boa, Anolis, and 

 Varanus) the chief bulk of the motor V nucleus is more ventrally 

 or rather peripherally placed (figs. 11 and 12). 



Nerves III and IV 



In Damonia the trochlear nucleus lies in the Sylvian gray 

 some distance rostrad of the level of its root exit. At this level 

 the floor of the aquaeduct is prolonged ventrad to form a deep 

 sagittal fissure between the fasciculi longitudinales mediales 

 (fig. 8). It thus happens that the trochlear nucleus which lies 

 upon the dorsal aspect of each posterior longitudinal fasciculus 

 is placed dorsad of the ventricular floor. It seems hardly neces- 

 sary to add that in figure 11, C, the distance from the upper 

 horizontal boundary line of the chart to the trochlear nucleus 

 indicates the approximate distance of the nucleus in question 

 from the ventricular cavity, not from the floor in the sagittal 

 plane. The same condition holds true in regard to the recon- 

 struction of the oculomotor nucleus. From its nucleus of origin 



Fig. 7 Damonia subtrijuga. Transverse section through the brain stem at 

 the exit level of the trochlear root. Abbreviations: Aq., iter; Dec. IV., trochlear 

 decussation; N. IV., trochlear nerve; Tr. b. dien., tractus bulbodiencephalicus 

 (de Lange, 12); Tr. th. b. sp., Tractus thalamo-bulbo-spinalis whose fibers are 

 intermingled with the preceding tract (De. Lange, I.e.); Tr. VIII., secondary 

 acustic paths. Other abbreviations as before. 



Fig. 8 Damonia subtrijuga. Transverse section through the brain stem at 

 the level of the trochlear nucleus. Abbreviations: Coll., caudal portion of col- 

 liculus; Dec. teg. dors., decussation of dorsal tectobulbar system (de Lange, 11, 

 12); Dec. teg. vent., decussation of ventral tectobulbar system; Nu. IV., trochlear 

 nucleus; Tectum, tectum opticum; Tr. t. b. vent., arcuate bundles of the tractus 

 tecto-bulbaris ventral is coursing ventromediad to decussate. Other abbrevia- 

 tions as before. 



