72 SUPPLEMENT ON 



slight preeminences to indicate them. In the perch it resem- 

 bles a reversed Phrygian cap ; in the mackerel the point is 

 turned forward ; in the tunny it is spread so as to cover nearly 

 the whole encephalon. In the chondropterygians the forms 

 of the cerebellum are still more varied, and sometimes its sur- 

 face is furrowed. The hollow lobes are oval, and their shell 

 is distinctly divisible into two layers or tunics. The fibres of 

 the external tunic are grayish, and run lengthways, ending 

 mostly at the optic nerve ; those of the internal are white and 

 transverse, lining the vault of the common ventricle which 

 these lobes contain. Upon the floor of this ventricle, are two 

 or four tubercles of a grayish substance, and placed before the 

 base of the cerebellum. The medulla oblongata is easily ob- 

 served extending forward, sending its external fibres into the 

 hollow lobes, and its internal into the anterior lobes. There is 

 a commissure which unites the anterior parts of the base of the 

 hollow lobes ; and behind it, and before the four tubercles, 

 opens the ventricle, corresponding to the third in the human 

 brain, and leading to the infundibulum and the pituitary gland. 

 By the sides of the infundibulum are the inferior lobes, usually 

 without a ventricle. These furnish fibres to the optic nerve, and 

 may therefore be regarded as the optic lobes in fishes. There 

 is a further singularity in the brain of these animals, consist- 

 ing in lobes placed behind the cerebellum, on the sides of the 

 fourth ventricle, varying greatly in different species, and 

 often very large, forming even five successive tumefactions. 



The olfactive nerves issue from the anterior tubercles ; they 

 vary greatly in form and composition in different species. 

 The optic nerves cross each other in front of the infundibu- 

 lum, without forming any further communication than through 

 the medium of cellular structure ; that on the right passes to 

 the left eye, that on the left to the right eye. Their medullary 

 structure sometimes presents the appearance of a ribbon folded 

 lengthways, so as to fill the tube of dura mater which contains 



