768 
MR. A. SANDERS ON THE ANATOMY OF THE 
quite untenable; not only its position behind tlie third ventricle, and the pineal gland, and 
at the mouth of the aqueduct, is against it, but Y. Baer had seen the ventricle in ques- 
tion in the cerebral lobes of the embryonic Teleostean, and Wilder (65) has pointed 
them out in the same tuberosities in the adult fish. The region round the fissure 
of the third ventricle would be the thalamus opticus ; and the torus semicircularis 
I would, with some doubt, refer to the corpus geniculatum externum. Fritsch has 
determined the deeper part of this torus as the corpus quaclrigeminum, and the super- 
ficial part as the thalamus opticus. The interpretation of the latter may be partly 
correct, but the relation of the former to the aqueduct of Sylvius forms a valid 
objection to its homology as given by this author. Of the remaining parts of the 
brain to be considered, the passage extending from the base of the third ventricle to 
the pituitary body is easily recognised as the infundibulum, and the glandular body 
into which it enters is clearly the pituitary body. The relation of the former to the 
latter is another instance of a foetal arrangement of higher animals, surviving as an 
adult arrangement in the lower. The hypoaria occupy strictly the position of the tuber 
cinereum, and the part called valvula cerebelli that of the valve of Wieussens, both 
being cases of structures better developed in animals of an inferior scale than in those 
of a superior. Finally, the cerebral lobes homologise the corpora striata, combined 
with the hemispheres of the brain ; and although I have not discovered in the species 
examined by me the ventricles in those bodies described by Wilder (65), yet I do 
not doubt that in other species they may exist. 
The presence of the fissure between the crura cerebri in the brain of fishes has some 
bearing on the theories lately propagated by Semper and Dohrn as to the origin of 
Vertebrata from the Annelida. The former author has not yet published any researches 
on the relation of the Vertebrate nervous system to that of the Invertebrata, but 
Dohrn ( 82 ) sees in the fissure belonging to the posterior end of the fourth Ventricle the 
remains of the space included in the commissures between the supra-oesophageal and 
infra-oesophageal ganglia of insects through which the oesophagus of the Invertebrate 
animal passes. There are several objections to this, one serious one being the amount of 
space to be filled up with nervous matter, of which no indications exist in the Teleostean 
brain. Mere likeness alone between the parts in the embryo fish and the insect would not 
suffice, as is shown by the curious resemblance between the cerebral cortex of a mouse 
and the tectum lobi optici of a Teleostean fish seen in a horizontal section, yet the optic 
lobes do not homologise the cerebral hemispheres. Another objection Fritsch has 
pointed out is, that if the oesophagus passed through this point, the jaws and mouth 
would have been supplied by a trunk from the supra-oesophageal ganglion, the fifth 
nerve being in advance. But if the theory be true, some other foramen must be found 
(if the oesophagus when it disappeared left any trace of its former existence) through 
which it could have passed ; such a foramen presents itself in the third ventricle, 
which, in conjunction with the infundibulum, forms an opening quite through the 
nervous tissue, being closed only below by the pituitary body and above by the pineal 
