Herrick, MorpJiology of Brain of Bony FisJics. ly^i 



could be selected than that furnished by the Teleosts. The 

 cerebellum is the most variable, and, at the same time, the 

 most characteristic segment of the brain. It varies greatly 

 even in the same group, yet the plan of structure is constant 

 and is even suited to characterize families and genera. The 

 structural peculiarity which is characteristic of all fishes is 

 the volvula (valvula of authors). This organ may be briefly 

 defined as a modification of that part of the roof of the aque- 

 duct of Sylvius which lies between the valve and the tectum. 

 Its structure is essentially that of the cerebellum, and it is 

 directly connected with the tectum cephalad. The extent to 

 which it develops seems to depend upon a variety of circum- 

 stances. When for any reason the cerebellum fails to acquire 

 the normal size the volvula may compensate therefor. The 

 size of the cerebellum as a whole (including volvula) is un- 

 doubtedly a function of the activity of the fish. Thus, for 

 example, the black-horse and buftalo-fish are allied systemat- 

 ically and live in the same streams, but the former is a very 

 active carp, while the latter is sluggish and massive. The 

 main portion of the cerebellum is of relatively the same size 

 in the two fishes, but a glance at Plate XIX, Figs, i and 4, 

 will convince one that the development of the volvula in the 

 black-horse corresponds to its active habits. In the typical 

 cases the volvula is but a forward fold into the optic ventricle, 

 but numerous secondary modifications obscure this primitive 

 simplicity. It may be well to refer to several illustrations at 

 this point, remembering that the cerebellum is mechanically 

 the least stable of the structures of the brain. The axial 

 portion of the organ is a thickening of the roof of the fourth 

 ventricle. The caudal and lateral aspects of the roof are 

 reduced to a membranous vekmi or infolded to form a plexu&, 

 while the cephalic boundary is altered to form the velum 

 anterior or volvula. Thus the only rigid attachment of the 

 cerebellum is that formed from the relatively small pedun- 

 culi. Rotations in all directions except cephalad meet with 

 no opposition, and the absence of closed cranial walls permits 



