Comparative Anatomy of the Brain. 579 



and olfactory lobes of fishes, become filled in and consolidated in mam- 

 mals, and it appears that the same thing happens to the cerebral lobes, 

 the cerebrum of the fish becoming the corpora striata of the mammal, 

 and forming a floor, over which the cerebral lobes are erected as a super- 

 structure, the lateral ventricles being entirely above the substance of 

 the floor and not in it ; nevertheless the cerebral lobes of the fish are all 

 to it that the cerebrum is to the mammal, because the composition of 

 their cortex of mingled nerve fibre and vascular neurine is the same as 

 that which forms the mammal convolutions. 



In the Pleuronectidse ( flat-fish ) neither the cerebral lobes, optic lobes 

 or hypoaria are symmetrical. ( See fig. 81.) The olfactory lobes are al- 

 ways distinct in fishes, and never united with each other by commissures. 

 They are often close together at their points of connection with the 

 cerebral lobes. Sometimes they do not join the cerebral lobes directly, 

 but are connected each by a stem of medullary matter called a Orus. 

 These crura are sometimes so short as to show only an indentation, as 

 in Lepidosiren (fig. 290 ); in other cases the crura are long, supporting 

 the olfactory bulbs on their ends, as in Sharks ( fig. 286, z ). The ol- 

 factory nerves start from the bulbs in front and form connection with 

 the organ of smell. There is no gray matter connected with the olfac- 

 tory nerve (or other nerves of conduction), but there is gray matter 

 contained in the crura mingled with the white fibres. As a rule, the 

 olfactory lobes and crura of fishes are solid. Lepidosiren and Shark 

 are exceptions, as noted above. 



The optic nerves and eyes of fishes are generally large. In Flat 

 fishes one optic nerve is smaller and shorter than the other. The optic 

 nerves are connected by filaments with the hypoaria, as well as with the 

 optic lobes. In ordinary osseous fishes, fibres from the optic nerves can 

 be traced into the cerebellum also. The optic nerves of fishes decussate, 

 or cross each other, but each nerve crosses over without giving any of 

 its fibres to the other, as in the mammals. (Fig. 81.) At the point of 

 crossing, the nerves are flattened, sometimes one nerve and sometimes 

 the other being on top. In most osseous fishes, the optic nerve consists 



FIG. 291. Plaited Optic Nerve of a Mullet. 



a. Optic nerve deprived of its sheath, showing its plaited 

 disposition. 



b Sclerotic Coat of the Eye, through which the nerve 

 passes. 



c. Retina, in which the nerve terminates. 



of a plate composed of membrane and nervous 

 matter, which is folded into a number of plaits, 

 a structure which usualty prevails throughout the 

 length of the nerve, from its cerebral attachment 

 FIG. 291 to tae eyeball ; in some instances the inner sur- 



face of the optic lobe is also folded, and in all, the plaits may be ob- 



