PISCES. 



993 



circumstance which proves that their brain does 

 not grow in the same proportion us the rest of 

 the body ; indeed Cuvier found the dimen- 

 sions of the brain nearly similar in individuals 

 of the same species, though one might be 

 double the size of the other. 



Fig. 526. 



firain of Perch, upper stir face. (After Cuvier.) 



a, cerebellum ; b, hollow (cerebral) lobes; c, ol- 

 factory lobes ; other letters as in the two following 



figures. 



The encephalon consists of a series of lobes 

 situated one behind the other (Jig. 526), con- 

 cerning the precise analogies of which no two 

 authors seem to be agreed. In the following 

 account of its general structure we shall there- 

 fore closely adhere to Cuvier's masterly analysis 

 of the organization of the brain of the Perc//, at 

 the same time, however, noticing the opinions 

 of anatomists, and the principal variations, ob- 

 servable in other Fishes, from the form of brain 

 selected for special description. 



The anterior pairof lobes (figs* 526,527, c,c) 

 invariably give origin to the olfactory nerves, and 

 consequently are very generally called the olfac- 

 tory lobes of the brain. Their surface is most 

 frequently smooth, but occasionally marked, as 

 for example in the Cod, with slight sulci. 

 Their relative size varies very much, but they 

 are generally, but not always, smaller than the 

 succeeding pair of lobes (b, b). They are con- 

 nected with each other inferiorly by a commis- 

 sure, which is sometimes double, and the in- 

 ternal fibres of the medulla oblongata may be 

 distinctly traced into their substance. 



In front of these olfactory lobes there are gene 

 rally one and sometimes two pairs of ganglia 

 (figs. 526, 527, 528, i, i) connected with the 

 origins of the olfactory nerves, which, when very 

 large, might be mistaken for additional lobes of 

 the brain. They are, however, never connected 

 by commissural fibres to their fellosvs of the 

 opposite side, and the olfactory nerve can be 

 traced along their under surface as far as the 

 proper anterior lobes of the encephalon (c, c). 

 Internally they frequently present a ventricular 

 cavity, which communicates beneath the ante- 

 rior commissure with that of the cerebral masses 

 next to be described. 



The second pair of cerebral lobes (jigs, 526, 

 527, 528, b, b) are of an oval form, and are re- 

 markable from the circumstance that they enclose 

 a wide cavity or ventricle, whence they have been 

 designated by some anatomists the hollow lobes 

 (Jig. 527, b). They consist, like the cerebral 

 lobes of the higher Vertebrata, of two layers, 

 which are generally easily separable; the outer 

 layer consisting of grey or cineritious matter, the 

 inner layer of the white or fibrous substance of 

 the brain, the fibres of the latter running trans- 



VOL. III. 



versely so as to line the roof of the ventricular 

 cavity, which is common to both sides of the 

 brain; for although the hollow lobes are united 

 to each other superiorly along the median line so 

 as to form a kind of corpus collosum with a me- 

 dian raphc, there is no septum between the two 

 sides. The fibres lining the ventricular cavity 

 seem to emerge from two semicircular bands of 

 grey matter (Jig- 527, /;, //) situated upon the 

 floor of the ventricle. 



Fig. 527. 



Brain of Perch, with the " hollow lobes" laid open, 

 and the cerebellum turned to the riyht side. (After 

 Cuvier.) 



Letters a, b, c, as in last figure ; g, supplementary 

 cerebellic lobes ; h, fibres lining the ventricular 

 cavity. 



At the bottom of the ventricular cavity there 

 are likewise, in osseous Fishes, two or four 

 tubercles of grey substance placed in front of 

 the ba>e of the cerebellum, and arching over 

 the canal which leads from the large cavity 

 contained in the hollow lobes into the ventricle 

 behind the cerebellum, which it is impossible 

 to consider as anything else but the representa- 

 tive of the fourth ventricle of the superior classes 

 of animals, and the canal of communication as 

 the " iter a tertio ad quartum ventriculum" of 

 the human anatomist, the tubercles themselves 

 being evidently the homologues of the tubercula 

 quadrigemina. 



The external fibres of the medulla oblongata 

 are easily traceable into the lobes we are now 

 considering, which, moreover, are connected 

 with each other by a broad commissure exactly 

 corresponding in situation with the anterior 

 commissure of the human brain. There can, 

 therefore, be no reasonable doubt that the 

 " hollow lobes " of the Fish's brain represent 

 the cerebral hemispheres of the encephalon of 

 Reptiles, Birds, and Quadrupeds, and, with 

 the data above given before us, it is not difficult 

 to point out the analogies of the remaining parts 

 not displayed in the figure. Thus, immedi- 

 ately behind the commissure is a passage 

 leading into the cavity, which corresponds to 

 the third ventricle, and which leads as usual to 

 the infundibulum and towards the pituitary 

 body that occupies its usual situation at the 

 base of the brain. 



The internal fibres of the medulla oblongata 

 may be traced forwards into these hollow cere- 

 bral lobes, in which they spread out as in the 

 higher animals. 



At the inferior surface of the brain, beneath 

 the " hollow lobes" just described, are two oval 

 protuberances (fig. 528, e, e), which are desi-- 

 nated by Cuvier the inferior lobes, between the 

 anterior extremities of which is situated the 



3 s 



