CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
949 
dibulum, and to join the optic tract. As to these fibres being derived from the 
posterior commissure, this is very unlikely, since this commissure is situated at a con¬ 
siderable distance away from this particular part.'' 7 The optic tracts are seen going 
upward and entering the roots of the tecta in two separate portions ; the inferior 
portion curves over the angle formed by their junction with the hypoarium and streams 
into the outer layer of those bodies, while the upper portion enters them by numerous 
small radiating bundles, which cross the bundles of the commissure of the tectum at a 
large angle and are lost in the internal layer; that is to say, the layer which corresponds 
to the third and fourth layers in the tectum of M. cephalus. The inferior portion 
(fig. 2) of the optic tract presents a peculiar arrangement of fibres which form a mass 
of decussations projecting above the dorsal edge of the optic tract in the central line, 
from which two or three bundles emerge, going obliquely upward and outward, crossing 
the crus cerebri to join the main body of the optic tract at the point where a complex of 
fibres is about to be formed, previous to their entrance into the tecta as above described. 
I could not make out with certainty that any of the fibres were derived from the tori 
semicirculares. These bodies are displaced further back in comparison to the tecta than in 
M. cephalus , or rather have retained to a greater extent their proper position, the latter 
having, as it were, moved forward, so that no part of the optic tract passes beneath 
their base. But there is a bundle of fibres (fig. 3) which passes downward along 
their inner edge, and appears to join the complex of fibres of the optic tract. 
The oculomotorius (figs. 4 and. 5) is derived from a ganglion situated on the floor of 
the ventricle of the optic lobe corresponding to its origin in M. cephalus. The only 
peculiarity about this nerve is that the fibres run obliquely backward towards their 
point of exit near the commissura ansulata instead of perpendicularly downward. 
The trifacial (fig. 7) presents the same number and derivation of its roots as in 
M. cephalus , and in addition derives a large number of its fibres from the anterior end 
of the tuberculum impar. 
The acusticus (fig. 7) also shows a complete concordance with the arrangement in 
M. cephalus. It passes forward in close contact with the central longitudinal column! 
and turns outward with the upper column of the trifacial, both forming an angle. 
Fritsch has suggested that the latter presents the marks of the facialis root in the 
brain of Mammalia, to which I would add that the acusticus root passing in close 
contact with it tends to increase the resemblance. 
The vagus (figs. 8, 9, 10) also presents roots of origin corresponding to those in 
M. cephalus , with the exception, perhaps, of the one from the cerebellum, which does 
not seem to be present. Some addition to its fibres is derived from the posterior part of 
* There does not appear to he anything extraordinary about this fact, since the trigontim fissum homo- 
logises with the tuber cinereum, and this part in the human subject sends fibres into the optic tract. 
(‘ Human Anatomy,’ Quacst, latest edition,) 
t figure 7 this does not show well, the parts being too small, but in the original preparation 
three distinct bundles of fibres are apparent here. 
