CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
757 
Although in the Plagiostomata the gigantic fibres discovered by Mauthnjer in the 
Teleostei are not present, and thus a difference is found to exist between these groups 
in the structure of the spinal cord, yet in Ceratodus, which perhaps presents an archaic 
form of nervous system, a form of nerve fibre is seen, which is in all likelihood the 
primitive form of this kind of fibre. In this nerve fibre, instead of one very large axis 
cylinder, there are several, which are all enclosed in a separate and distinct medullary 
sheath ; they occupy a position in the spinal cord and medulla oblongata correspond¬ 
ing- to Mauthner’s fibres. 
The ventral columns on arriving at the fourth ventricle form two distinct longi¬ 
tudinal projections in the central line of the floor of the same ; anteriorly these pro¬ 
jections become smaller and disappear at the anterior end of the fourth ventricle; 
they disappear as such, but the columns themselves are traceable farther forward 
much diminished in size. Along the floor of the aqueduct of Sylvius they gradually 
break up into a number of smaller bundles which recede from the floor of the aqueduct 
toward the outer side. They can be traced, much diminished in size, into the ventral 
part of the posterior commissure. The lateral columns also diminish greatly in the 
number of fibres which they contain in their course forward. Those bundles which are 
internal and nearer the ventral columns become lost in the region of the posterior 
commissure, while some of the more external seem to join the transverse fibres which 
form the second layer of the optic lobe. Those between the two disappear in the 
region above the liypoarium, some crossing the fibres of the crura cerebri, which dis¬ 
appear in the same region. Rohon * describes these columns as the continuation 
backward of the pedunculi cerebri, with which he thinks they are continuous, and he 
identifies the ventral columns with a system that he has traced in the cerebrum,! and 
which he has homologized with the fasciculus longitudinalis tegmenti posterior. This 
sort of thing is a mere tour de force of the imagination; these columns may horno- 
logize in a general sort of way, but to pick out the particular fasciculi in the human 
brain to which they correspond is going a little too far. I am unable to confirm his 
assertions, as neither of these connexions were to be traced in my sections (fig. 18 ). 
Deep Origin of the Cerebral Nerves. 
First pair.—The olfactory nerves are derived from the olfactory lobe, a description 
of which has already been given. 
Second pair.—The optic nerves form a distinct and well-formed chiasma. They do 
not show the pectinated arrangement characteristic of the Teleostei. At the lower 
portion of the chiasma the fibres of the nerve of one side intersect those of the other 
in layers as they cross, like the intertwined fingers of the two hands ; but dorsad, more 
than half of the nerve of one side crosses over that of the other, forming a simple 
* Op. cit., pp. 72 and 77, fig. 51. 
j Op. cit., p. 79, fig. 42. 
