FISHES. 327 



The number, forms, and relative proportions of these tubercles vary 

 according to their genera. 



In certain fishes, as the carp, the anterior pair is long, and directed 

 backwards, being bent like a ram's horn. 



In others as the mackarel, the posterior pair is greatest ; it forms 

 a curve forwards, and appears to fold itself like an intestine. 



In the tunny there are as many as three tubercles on each side, 

 some placed beside others, and resembling so many folds of in- 

 testines. 



These are wholly wanted in the chondropterygians where we can 

 no longer trace distinct fibres on the inner surface of the hollow 

 lobes. 



In the greater number however of fishes, especially in the perches, 

 pikes, herrings, cods, &c, the internal tubercles amount to four, 

 varying but little in size. 



The nerve of the fourth pair arises behind tbe hollow lobes, 

 the tubercles which they contain, and the furrow which separates 

 them from the anterior base of the cerebellum ; sometimes it arises a 

 little on the side, but not at all below, as has been represented. 



It is difficult to trace the medulla oblongata in its course towards 

 the anterior portions, and to see it passing, after leaving the cerebel- 

 lum, by its more external fibres into the hollow lobes (b, bj, and by its 

 more internal ones into the anterior lobes. 



These latter lobes when they are not perfectly soldered together, as 

 often happens in the rays and squalus, communicate with each other, 

 at least by one and sometimes two commissures (A). The proportion 

 between them is various ; very commonly they are smaller than the 

 hollow lobes ; they are very large in the eels : but in the rays and 

 squali their superiority is quite enormous. 



The ganglions or tubercles (7, t) which are sometimes before the 

 anterior lobes, these lobes being sometimes found to be two pairs, as 

 in the eels, are not joined to one another by a commissure, but each 

 is united to the lobe before it ; and we are enabled to trace the olfac- 

 tory nerve beneath the inferior surface as far as the commissure (k) of 

 the anterior lobes (c, cj. 



There is always a commissure (in) for joining the anterior portions 

 of the base of the two hollow lobes ; and it is behind this and before the 

 four tubercles contained in these lobes, that the ventricle opens, which 

 is analogous to the third ventricle in man, and which leads as usual 

 to the infundibulum, and towards the putuitary gland on the inferior 

 surface of the brain. 



The two lobes (e, e) which we have called the inferior, are seen at 

 the sides of the infundibulum. They are generally tolerably large, of 

 the shape of a kidney, and a ventricle is seldom found in their in- 

 terior. 



When however, it is found, it communicates w r ith the third ven- 

 tricle, and through it with the great ventricle Avhich is common to the 

 two hollow lobes. 



The inferior tubercles (e e,) furnish very evidently the fibres of the 

 optic nerve, and behind them, and also in the furrow by which they 



