NERVOUS SYSTEM. (COMPAHATIVE ANATOMY,) 



619 



ganglion, situated in the median line ; in the 

 perch, gurnard, cod, pike, roach, chub, carp, 

 and dace, of a pair of ganglia, and this is the 

 most usual arrangement. In the skate, one of 

 the Plagiostome fishes, where the brain is alto- 

 gether more highly developed, there is one large 

 ganglion or cerebral mass; it is solid, but in 

 some of the sharks it contains a cavity. From 

 these eminences, whatever be their number, the 

 olfactory nerves (processes or lobes) arise Cfigs. 

 350 and 351, k, /c), which, running together in 

 an osseous canal for some little distance, 

 diverge, and form large tubercles on the cribri- 

 form plate of the ethmoid bone (Jig. 352, /) ; 

 from these tubercles nerves arise, which are 

 distributed to the pituitary membrane of the 

 nose. 



2dly. The optic lobes, or second cerebral 

 mans (jigs. 351 and 352, c, 6), which Collins,* 

 Monro,* Camper,* Ebel,* Treviranus,* and 

 Cuvier,* considered as analogous to the cere- 

 bral hemispheres of the mammalia, but which, 

 with Serres,t Desmoulms,t Arsaky,f Carus,f 

 and Tiedemann,f I consider as analogous to 

 the tubercula quadrigemina, are generally of 

 large size in fishes, and contain internal tuber- 

 cles and cavities, which communicate with the 

 fourth ventricle. These masses may be said to 

 arrive in this class at their maximum of deve- 

 lopement, and we may recollect to have traced 

 out their first rudiments in the cerebral ganglion 

 of the Gasteropodous Mollusca, and to have 

 noticed their successive complication of deve- 

 lopement in the varied classes of articulated 

 animals. In the lamprey these optic lobes are 

 larger and more developed than any other parts 

 of the brain ; and this is what we should be 

 led to expect from the low organization and 

 vermiform nature of these Cyclostomous fishes : 

 they contained in their interior a cavity. In 

 the eel, perch, cod (fig. 351, c), gurnard, 

 mackarel, pike, roach, chub (fig. 352, 6), carp, 

 and dace, true osseous fishes, the optic lobes 

 are well developed, and, excepting in the eel, 

 much larger than the olfactory tubercles before 

 them ; they are hollow, and contained tubercles, 

 which vary in number, size, and position. In the 

 eel there are two of these tubercles in each hollow 

 lobe, equal in size, and situated posteriorly; in the 

 cod there are also two, the outermost being the 

 largest, and smooth, the inner one being smaller, 

 and constricted in the middle (jig. 351, e); in 

 the mackarel there are two, the anterior one 

 being exceedingly small, the posterior much 

 larger, slightly convoluted, somewhat resemb- 

 ing the Greek letter y ; in the pike there are 

 two, and the floor of the cavity had a striated 

 appearance ; in the roach there is only one 

 large tubercle; and in the carp there are two, 

 the anterior being rather long, and passing 

 backwards in a curved manner. From these 

 lobes the optic nerves ('Jig. 351, /) arise, and 

 cross each other, without, however, any other 

 connexion than mere cellular tissue. The third, 

 fourth, and sixth pairs have also their origins 

 from these ganglia. 



[The optic lobes have a direct relation in 



* Op. cit. 



f Op. cit. 



point of volume with that of the eyes, and in 

 the pleuronecta, in which the eyes are of une- 

 qual size, Gottsche states that the optic lobes 

 are unequal.] 



The tubercles situated on the inferior surface 

 of the brain, and immediately beneath the 

 optic lobes just described, are generally of 

 small size, and seldom contain a cavity; be- 

 tween them are the infuiidibulum and pituitary 

 gland, generally of very large proportional size. 

 Respecting their analogies and names, very 

 much difference of opinion exists. Haller 

 termed them the inferior protuberances of the 

 olfactory nerves ;* Cuvier considered them as 

 the true optic lobes ;f Dr. Grant calls them 

 the cerebral hemispheres, and supposes they 

 are the representatives of those parts in the 

 higher animals ;J Serres considers them appen- 

 dages to the optic nerves, and analogous to the 

 tuber cinereum ; Vicq d'Azyr,|| Arsaky,^" and 

 Carus, consider them analogous to the corpora 

 mammillaria of higher animals :** Tiedemannf j- 

 does not decide upon this point, but judges 

 (from the situation and form of the tubercles) 

 that the latter hypothesis is the more probable 

 one. 



3dly. The cerebellum, or third cerebral mass 

 (Jig. 35 1,/; Jig. 352, c), is but imperfectly 

 developed in Ashes ; it is generally of a round 

 form, and covers in the cavity formed by the 

 divergence of the two cords of the spinal mar- 

 row and an enlargement of its canal, the fourth 

 ventricle. In the lamprey there are scarcely 

 any traces of a cerebellum, a thin transverse 

 band of medullary matter being all that stands 

 for it; the fourth ventricle is here, therefore, 

 quite open and exposed. In the eel it is large, 

 and of a rounded form ; in the perch its sum- 

 mit is directed backwards ; in the mackarel, 

 forwards ; in the cod (fig. 351, J") and pike it 

 consists of a tongue-shaped lobe ; in the gur- 

 nard, roach, chub ffig. 352, c), and dace, it is 

 round, and of moderate size ; in the carp it is 

 also of a rounded form, but immediately be- 

 hind and below it is situated another ganglion 

 of smaller size, on each of which is a larger 

 ganglion, principally destined for the origin of 

 the branchial nerves, thus rendering the struc- 

 ture of the cerebellum very complicated, and 

 its size very voluminous. In the Plagiostome 

 fishes the cerebellum is much more highly de- 

 veloped. In the skate it is of large relative 

 size, furnished with two lateral appendages, 

 the commencement of lateral hemispheres, on 

 the external surface of which transverse and 

 longitudinal stria were developed. 



On reviewing these statements of the nervous 

 system of the fishes, we observe two things 

 that more particularly mark its low organization 

 the equality and the horizontal position of the 

 brain and spinal marrow. In fact, as regards 



* Opera minora, vol. ii. 



t Anatomie Comparee. 



J Lectures on Comparative Anatomy. 



Anatomie Comparee du Cervcau. 



Opera minora. 

 ^[ De piscium cerebro. 

 ** Anatomie Comparee. 

 ft Anatomy of the Fatal Drain. 



