994 



PISCES. 



pituitary body (f). These inferior lobes are 

 generally of considerable size, of an oval or 

 kidney shape, and sometimes, but rarely, con- 

 tain ventricular cavities, which communicate 

 with the third ventricle, and through it with 

 the great ventricles contained in the " hollow 

 lobes;" they furnish fibres of origin to the 

 optic nerves, and it is from the fissure between 

 them and the medulla oblongata that the nerves 

 of the third pair take their origin. 



Fig. 528. 



Brain of Perch, lower surface. (After Cuvier.) 



Letters a, b, c, g, h, as in preceding figures ; e, 

 inferior lobes ; /, pituitary body i, swelling at com- 

 mencement of olfactory nerve , n, optic nerves j o, 

 olfactory ; p, q, r, s, t, encephalic nerves. 



The pituitary body (Jig. 528, f) occupies its 

 usual position on the base of the brain at the 

 extremity of the infundibulum; it is generally 

 of large size in Fishes, and is often connected 

 with membranous and vascular appendages of 

 various forms, a circumstance which is, how- 

 ever, most remarkable in the cartilaginous 

 Fishes. Occasionally, as for example in the 

 Lophius and the Haddock, the infundibulum 

 is prolonged into a slender filament, and the 

 pituitary body is removed very far forward ; 

 but the uses of these parts are as problematical 

 in Fishes as in the other classes of Vertebrata. 



The cerebellum (Jigs. 526, 527, a) in osseous 

 Fishes is of considerable size, but consists of 

 the median portion only, no lateral lobes being 

 as yet developed, or at least they are only indi- 

 cated by slight eminences. Its shape is gene- 

 rally that of a blunted cone, the apex of which 

 is bent backwards; but there are instances, as 

 for example the Mackarel, in which it is di- 

 rected forwards, and sometimes it extends so 

 far forwards as to overlap all the rest of the en- 

 cephalon. Owing to the deficiency of the la- 

 teral lobes of the cerebellum there are of course 

 no traces of a pans Varolil. 



Behind the cerebellum, on each side of the 

 fourth ventriclej, and sometimes even covering 

 that cavity, are certain supplementary lobes 

 (fig. 527, g ) which would seem to be peculiar 

 to Fishes, and which are very variable in their 

 proportions, forms, and connections. In most 

 osseous Fishes they consist of two protuber- 

 ances or swellings of the sides of the medulla 

 behind the cerebellum, which touch each other 

 along the mesial line or are united by a com- 

 missure. 



In the Cyprinidae their volume is so consi- 

 derable that they cover the greater part of the 

 medulla oblongata, and their sides are furrowed 

 with transverse striae. In the Grey Mullet they 

 are also very large, and their surface is marked 

 with tortuous sulci, giving the appearance of 

 cerebral convolutions. It is, however, in the 

 Triglae or Gurnards that these lobes are most 

 largely developed, amounting in number to as 



many as five on each side, and occupying a 

 space equal in length to all the rest of the 

 encephalon, and extending backwards as far 

 as the second vertebra. It is from the last 

 of these lobes that the second pair of spinal 

 nerves is given off, which in this genus supplies 

 the free rays situated in front of the pectoral 

 fins.* 



In the chondropteryginous Fishes the struc- 

 ture of the encephalon offers many remarkable 

 peculiarities. Thus, in the Rays and Sharks the 

 proportionate size of the olfactory lobes is enor- 

 mous, and instead of simply having a commis- 

 sural communication with each other, they are 

 consolidated into one mass. The hemispheres 

 enclose a capacious ventricle, but there are no 

 distinct fibres visible upon its inner surface, 

 neither are the representatives of the tubercula 

 qtiadrigemina of osseous Fishes apparent. The 

 cerebellum is of great relative size, but of very 

 variable form in different species ; and not 

 unfrequently it is divided into lamina by deep 

 transverse sulci. The supplementary lobes 

 behind the cerebellum are represented by folds 

 or cords of nervous matter, which are prolonged 

 from each side of the posterior edge of the 

 base of the cerebellum and run backwards 

 along the margin of the fourth ventricle. 



The brain of the Tunny ( Thynnus vu/g. 

 Cuv). is remarkable for the extent of the cere- 

 bellum, and the complication of the internal 

 tubercles. The olfactory nerves are small and 

 oval. The hollow or cerebial lobes are of very 

 great size, and nearly spherical, with a lateral 

 fissure inferiorly. On opening them, instead of 

 the tubercles generally met with in Fishes, 

 there is found on each side a mass divided into 

 three lobes, which are themselves grooved with 

 a fissure, so that the whole resembles a cylinder 

 or cord having six folds, twelve in all. The 

 cerebellum is larger than the rest of the ence- 

 phalon, and, arising from the medulla oblon- 

 gata, curves forwards, overlaying both the 

 hollow lobes and the olfactory lobes even as far 

 as the anterior extremity of the latter, its breadth 

 being little less than half its length. At the 

 posterior part of its base there is on each side 

 a rounded protuberance, different in character 

 from those enlargements which are frequently 

 met with in other Fishes at the commencement 

 of the medulla oblongata. 



Nervous system. The olfactory nerves (Jig. 

 529) arise from the olfactory lobes of the brain, 



* The various names applied by different authors 

 to the different parts described above are calculated 

 to create great embarrassment and confusion. Thus, 

 Haller in his ' Physiology,' and likewise in his 

 ' Opera Minora,' calls the lobes (c, c) anterior olfac- 

 tory tubercles, the lobes (e, e) inferior olfactory tu- 

 bercles, the hollow lobes (b, b) optic thalamus, &c. 

 M. Arsaki, in his thesis ' De cerebro et medulla 

 spinali piscium,' calls the hollow lobes (b, b) tuber- 

 cula quadrigemina, and regards the anterior lobes 

 (c, c) as the representatives of the hemispheres. 

 M. Weber, in his ' Anatomia comparata nervi sym- 

 pathetici,' whilst he recognizes the hollow lobes 

 (b, b) as the cerebral hemispheres, regards the 

 cerebellum (a) as the analogue of the tubercula 

 quadrigemina, and the supplementary lobes (ff, g ) 

 at the commencement of the medulla oblongata as 

 representing the cerebellum. 



