Herrick, Morpliology of Brain of Bony Fislies. 33c; 



The Thalamus. — In general, it may be admitted that Baer 

 correctly characterized this region in the statement, " Es 

 sieht so aus, als ob das Mittelhirn das Zwischenhirn unter- 

 driickt habe." The strong optic nerves and tracts serve to 

 constrict the organ greatly and obscure the original form. 

 In some cases the origin of the epiphysis and habena are 

 visible from above, while in others, like the black-horse, the 

 mesencephalon is thrust forward by the great development 

 of the volvula, so as to cover much of the cerebrum itself. 

 Apparently, as a result of the constriction of the middle of 

 the thalamus, it is forced caudad, and, to supply the requisite 

 nervous material, develops the hypoaria or inferior lobes, 

 which, instead of representing the mammillary bodies, seem 

 to contain the homologues of the displaced walls of the 

 median part of the thalamus. The saccus vasculosus marks 

 the caudad extension of the thalamus. It seems necessary 

 to recognize three parts of the thalamus, as follows. The 

 prcethalamus : This includes the ventral median region 

 caudad of the anterior commissure and ventrad of the hippo- 

 campal lobules. It is quite distinct from the adjacent parts 

 of the cerebrum, and contains the various tracts passing 

 from it toward the lower parts of the brain. The mid-thala- 

 mus is that part which corresponds to the principal part of 

 the organ in reptiles and bears the habena. Its caudal limit 

 may be recognized in the inferior commissure. Caudad of 

 the this, and extending as far as the saccus vasculosus, is a 

 portion which is covered dorsad by the optic lobes, and 

 bears laterally the hypoaria. This may be termed the fiost- 

 thalainus . 



There is no difficulty in recognizing these divisions in 

 any of the Teleosts examined. 



Mesencephalon. — The topography of the optic lobes differs 

 from that of reptiles only by reason of the greater or less 

 invasion'of the optic ventricle by the volvula. Transections 

 of the mesencephalon of the eel cephalad resemble corre- 

 sponding sections of the turtle closely. There is a slight pro- 



