BEAIX OF VEETEBEATA. 



505 



simple in the Cyclostomata, and in the Selachii presents indications 

 merely of its divisions. It is not developed to any great size 

 except in the Teleostei. The succeeding mesencephalon is small in 

 the Myxinoids, but larger in Petromyzon. In the Selachii it ap- 

 pears to be united to the thalamencephalon, while the portion which 

 corresponds to it, in position at any rate, is regarded as the cerebellum. 

 This portion is very greatly developed, so much so, indeed, that it 

 covers over those parts of the brain that lie in front of and behind 

 it (Fig. 281, b). In the Teleostei this part of the brain is pro- 

 portionately more developed, and sometimes has the form of a 

 protuberance directed forwards or upwards. The rest of the brain, 

 which lies behind the mesencephalon, must be regarded as one piece. 

 It is of great importance as being the region from which 

 most of the cerebral 

 nerves take their origin. 

 Its roof is unequally de- 

 veloped. That is to say, in 

 the hinder, and larger, por- 

 tion, it is soon atrophied, so 

 that the internal cavity 

 (sinus rhomboidalis), which is 

 widened out anteriorly, is 

 only covered over by mem- 

 brane. In the Selachii and 

 Chimasrse the edge of this 

 sinus is thickened anteriorly 

 (lobi nervi trigemini). It is 

 simpler in the Ganoidei and 

 Teleostei. But in all Fishes it 

 is continued mesially into a 

 transverse lamella (Fig. 282, 

 b c), which covers in the sinus 

 anteriorly, and has the mesen- 

 cephalon projecting over it, 

 when this portion is very 



Fig. 282. Brain of Polypterus bichir. 

 A From above. B From the side. C From 

 below, h Lobi olfactorii. g Prosencephalon. 

 / Thalamencephalon. d Mesencephalon, be 

 Metencephalon. a Myelencephalon (medulla 

 oblongata), ol N. olfactorius. o N. opticus 

 (after J. Miiller). 



large. This transverse la- 

 mella appears to correspond to the cerebellum of the higher Verte- 

 brata, while the base and sides of the sinus are formed by the myel- 

 encephalon (medulla oblongata). As we pass from the Selachii 

 to the Teleostei we note that the medulla oblongata diminishes in 

 size; in many Sharks it forms the largest portion of the brain. 

 And this corresponds to a primitive stage, in which it and the 

 mesencephalon form the largest part of the whole brain. 



When it is still more developed, swellings may be observed 

 on the sides of the fourth ventricle ; these are set in series, and 

 correspond to the points of origin of the roots of the vagus 

 (lobi nervi vagi). The lobi electrici of the Torpedines are dif- 

 ferentiations of this kind, which, however, unite above the narrowed 

 ventricle (cf. Fig. 277, iv). 



