CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
7G3 
Von Baer’s theory, although subsequent researches proved it to be untenable, was 
nevertheless much more reasonable, for there was some ground for it, in the projection 
of a part of the cerebellum into the ventricle of the optic lobe. 
Next the dilemma, which our author propounds on p. 99, as to the trochlearis, 
has no existence except in the imagination, This nerve does not arise so far back 
as he supposes ; if one inspects his fig. 49 it will be at once seen that one has to do 
with a section taken behind the crura cerebelli ad medullam, and that the structure 
represented could not be the roots of this nerve. The true point of decussation of 
this nerve has been described in the account of the origin of the nerves. 
There is no question here of any physiological consequences, either good or bad, 
in consequence of its fibres passing through the optic tract, since the case does not 
occur, the two sets of fibres being some considerable distance apart. 
In addition to these considerations I may cite Rabl-Ruckhard,* who states 
that at no time during the development of the embryo does the thalamencephalon 
pass or project behind the point of attachment of the epiphysis cerebri. Neither does 
Balfour t give any indication of any such occurrences. 
If we should abandon the landmarks of homology as Rohon and Fritsch would 
have us do, we shall be plunged into a sea of doubt ; science would be reduced to a 
mere system of guesswork, and we should become like those natives in South Africa 
whom a recent traveller described, as amusing themselves over the camp fire by 
discussing the question as to which fingers of their hand the toes of an ostrich 
corresponded. 
Explanation of Plates. 
The following letters have the same signification throughout :— 
O o O 
ax. 5 
ci.c.ce. 
a. l. 
aq.Sy. 
b. 
c. a. 
c.c. 
c.ca. 
c.cbl. 
c.cbl.c. 
cbl. 
ce. 
Ascending root of trifacial. 
Anterior commissure. 
Anterior lobe of the cerebellum. 
Aqueduct of Sylvius. 
Nervi branchiales. 
Commissura ansulata. 
Crura cerebri. 
Central canal of the spinal cord. 
Crura cerebelli ad medullam. 
Crura cerebelli ad cerebrum. 
Cerebellum. 
Cerebral lobes. 
* LoC. dt. 
I ‘ A Monograph on the Development of Elasmobranch Fishes.’ London, 1878. 
5 E 2 
