CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
741 
homology of the brain in fishes on a new basis firmly centred, as he supposed, on embryol- 
ogy. His opinion has been endorsed by no less an authority than that of Gegenbaur 
(67), which even in the ‘ Grundriss ’ just published he has not seen fit to alter. The 
author also informs his readers that Y. Baer gave to his theory a verbal assent. 
This being the case, his opinion, perhaps, merits a more detailed consideration. 
With him, therefore, the cerebral lobes are the “ vorderhirn,” the optic lobes are 
the “ zwischenhirn ” (thalamencephalon), the cerebellum the “ mittelhirn ” (mesence- 
phalon), and the pons mammillaris the “ hinterhirn,” and the posterior part of the 
medulla oblongata the “ nachhirn.” It is thus seen that the cerebellum in many 
fishes is reduced to a mere bridge of granular material, covering the narrow part of 
the fourth ventricle. 
Against this scheme Stieda (64) has placed on record several very important 
considerations : 1st, that the optic nerve, which in Amphibia and Birds arises from 
the mesencephalon (corpora quadrigemina), would, according to this mode of interpreta- 
tion, arise from the thalamencephalon ; 2nd, then the trochleares would arise wrongly 
between the thalamencephalon and the mesencephalon ; 3rd, the thalamencephalon 
of fishes would not correspond to that of the amphibia, where it is a small flattened 
segment open above ; but in fishes, if Miklucho-Maclay be right, it becomes deve- 
loped into a large closed segment, which is often paired ; 4th, the pineal gland would 
be wrongly placed. 
With regard to the third objection, Miklucho-Maclay appears to have overlooked 
the real thalamencephalon, which is the territory surrounding that narrow fissure, the 
greater part of which is in front of the optic lobes, so that in point of fact he searched 
for the “ zwischenhirn ” too far back. 
The microscopic structure of the “ mittelhirn ” of Miklucho-Maclay, as was also 
pointed out by Stieda, and as will be seen in the following pages, so entirely corre- 
sponds to that of the cerebellum in higher Vertebrates, that any comparison of it 
with any other part of the brain is wholly precluded. 
The brain has hitherto been considered from above. On examining its lower surface 
the following structures are to be observed from before backwards, viz., in front, the 
entrance of the optic nerve, behind which is situated the pituitary body (fig. 1, pi., 
figs. 3 and 4) or hypophysis cerebri, placed on a slight eminence, the trigonum fissum 
of Gottsche ; on each side a large tuberosity (fig. 1, hy., figs. 3, 4, 5, and 6), the lobus 
inferior, or, as I think preferable, the “ hypoarium,” used by Owen in his ‘Anatomy 
of the Vertebrata : ’ a term having the same meaning but being less clumsy. 
The posterior parts of the groove (fig. 5, s.v.) indicating the separation between the 
two hypoaria is occupied by a vascular sac, the “saccus vasculosus ” or “ heematosac.” 
This sac in Mugil cephalus is not visible from the surface, being contained in a sort of 
chamber formed between the contiguous walls of the two hypoaria. 
Haller appears to have looked upon the hypoaria as having relation to the sense 
of smell, for he termed them “ olfactoria inferiora ” or “ tubercula reniformia.” 
