CRANIAL NERVES OF SIREN LACERTINA 261 
The trochlear nerve arises externally from the extreme posterior 
border of the midbrain. The two trochlear nerves decussate 
along the dorsal border of the cerebellar commissure (decussatio 
veli), the ‘commissura intertrigemina’ of Bindewald (711) (fig. 
16, dces.). Sagittal sections of the roof of the midbrain, a little 
Figs. 16to19 Sagittal sections through the posterior part of the optic tectum, 
the cerebellar commissure and the decussation of the trochlear nerve. 
Fig. 16 Section near the middle line. 50. 
Fig. 17 Cerebellar commissure and trochlear nerve of the same section more 
highly magnified. 250. 
Fig. 18 Same structures situated more laterally. 250. 
Fig. 19 Similar to figure 16, but of an adult Necturus. 50. 
at one side of the middle line, show the two nerves distinct from 
each other, but at the middle line their individual identity is not 
so evident (fig. 17). Near the point where a trochlearis passes 
from the brain wall of its respective side, the tract of the other 
JOURNAL OF MORPHOLOGY, VOL. 24, No. 2 
