CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
761 
along the external edge of the medulla to its ventral edge, whence it passes over to 
the corresponding point on the opposite side. The first-mentioned transverse commis- 
sures extend at internals as far forward as the posterior end of the ganglion of the 
motores oculorum ; the most anterior bundles become smaller, and pass above instead 
of beneath the central longitudinal column. Those situated in the region of the crura 
cerebelli could not be traced directly into them. 
Commissura Ansulata. (Figs. 1, 2, and 5, c.a.) 
There is a double system of transverse commissures at the region of the origin of 
the motores oculorum and the posterior end of the tectum which are derived from the 
anterior part of the latter body. They are the commissura ansulata of Gottsch (34). 
Commencing the description of the outer circle of these commissures from the system of 
transverse fibres passing across the base of the fornix, we can trace the fibres through the 
fifth layer of the tectum, then by numerous bundles into the torus, where they form the 
system of the crura lobi optici, then downwards and backwards along the external edge 
of the medulla to its ventral surface, on the roof of the furrow between the medulla and 
the hypoarium, where after giving a few fibres to the motor oculi of the opposite side, 
they cross over to return to the region whence they started. The other part of this 
double commissure follows the same course as the last, until it attains the lower part 
of the external edge of the medulla, and there it turns inwards towards the mid-line ; 
it then crosses obliquely to the other side, and attains the inner part of the torus, near 
the point of junction of the aim of the valvula cerebelli with that body, and extends 
nearly as far forward as the anterior end of the optic lobe. The latter commissure 
extends in the medulla to about one quarter the length of the space occupied by the 
former ; they are derived from bundles of longitudinal fibres which first appear on the 
lateral borders of the medulla oblongata opposite the narrow part of the forth ventricle 
(fig. 9, c.a.). 
Some writers compare this commissure to the pons varolii ; this can scarcely be a 
true homology, since none of the fibres come from the cerebellum. It seems to me 
more probable that they homologise the decussation of the anterior pyramids ; they 
resemble to a slight extent the figures of that decussation given by Lockhart 
Clarke (70). Baudelot (61) was also of this opinion; he termed them “com- 
missure anterieure des pyramides;” he appears only to have been acquainted with the 
first described one. The objection that occurs to me is that they are too far forward 
m front of, instead of behind the fourth ventricle; they also come from the optic 
lobes. 
At the anterior end of the floor of the optic lobes are two transverse commissures, 
one superficial and one deep). The superficial commissure (fig. 3, p.c.) passes over the 
posterior part of the third ventricle, and the passage that puts it into communication 
with the ventricle of the optic lobe. This passage is, in fact, the anterior end of the 
5 e 2 
