326 FISHES. 



rather \vhich are sometimes distinguished from them by a constriction. 

 Before deciding between these opinions, it is necessary to enter into 

 larger details upon the form and composition of these parts, and upon 

 the principle variations which they undergo in different species. 



The relative size of the cerebellum (a) is also considerable, and it 

 often surpasses in volume the parts situated in front of it ; its lateral 

 lobes either do not exist, or only form very slight protuberances. 



In osseous fishes, for example in the perch, its form is frequently 

 that of a soft cone, the summit of Avhich is curved backwards, nearly 

 like an inverted phri'gian hat; and nevertheless there are, also, somelike 

 the mackarel, in which its summit bends foi-\vards ; and others, as the 

 tunny, in which it extends backwards and forwards in such a manner 

 us to receive all the remaining jiart of the brain. 



In the chondropterygians, it assumes very different forms and vol- 

 umes ; sometimes it is almost reduced to a transverse bar, as in the 

 sturgeon and lampreys, sometimes round or oval, or lobed, and very 

 voluminous, as in the ray, and more especially in certain squalus. 



Its surface is sometimes transversely fuiTowed, particularly in 

 several of the squalus and the tunny, and even when it is smooth, as 

 is generally the case, there is seen in the interior of its substance a 

 medullary axis, wliich gives out ramifications of the same nature in 

 the cortical substance, and which is hollowed by a cavity that commu- 

 nicates mth the fourth ventricle. 



The hollow lobes (b, h) placed immediately before the cerebellum, 

 and upon the nature of which there are different opinions, are of an 

 oval form. 



In the greatest number of osseous fishes, their envelope presents 

 two layers, often very easily separated, the external one is grey, the 

 internal one white. 



The fibres of the external layer, obliquely directed from behind 

 forwards, terminate for the most part in the optic nerve ; but they co- 

 operate in its formation w'ith other fibres arising, some from the in- 

 ferior lobe, others from the medulla oblongata, and some even, as 

 may be seen in the rays, from the anterior lobe. 



The fibres of the internal layer of the hollow lobe, much more easily 

 discovered than the others in osseous fishes, are directed transversely, 

 and line the arch of the common ventricle enclosed in the hollow 

 lobes. 



They appear to arise from a semi-circular pad {h li, fig. ix), of 

 greyish matter, which occupies on each side the base of this ventricle, 

 as the fibres of the roof of the hemispheres in the human subject 

 rise from notched bodies. 



The arches of the hoUow lobes unite, in a median line, which forms 

 a species of callous body, and a ridge projecting inwards ; but there 

 is no complete septum. 



Upon the floor of this ventricle are seen (in the osseous fishes) two 

 or four tubercles of a greyish substance {d, fig ix.) and placed before 

 the base of the cerebellum upon the aqueduct, which leads from the 

 ventricle of the hollow lobes into that which is under the cerebellum, 

 and at its posterior, which corresponds to the fourth ventricle in the 

 higher classes. 



