100 



FISHES. 



most other points of its organisation. The hemispheres 

 form the largest part of the brain ; they are coalescent, as in 

 Sharks, but possess two lateral ventricles, the separation being 

 externally indicated by a shallow median groove on the upper 

 surface. The olfactory lobes take their origin from the upper 

 anterior end of the hemispheres. Epiphysis and hypophysis 

 well developed. The lobi optici are very small, and remote 

 from the prosencephalon, their division into the lateral halves 

 being indicated by a median groove only. The cerebellum is 

 very small, overlying tlie front part of the sinus rhomboidalis. 

 The brain of Chondropterygians (Fig. 43) is more developed 

 than that of all other fishes, and distinguished by M^ell-marked 

 characters. These are, first, the prolongation of the olfactory 

 lobes into more or less long pedicles, which dilate into great 

 ganglionic masses, where they come into contact with the 

 olfactory sacs ; secondly, the space which generally intervenes 



between prosencephalon 

 and mesencephalon, as in 

 some Ganoids ; thirdly, 

 the large development of 

 the metencephalon. 



The hemispheres are 

 generally large, coalescent, 

 but with a median, longi- 

 tudinal, dividing groove. 

 physis ; g, nerviis opticus ; h, bemi- Frequently their surface 



sphere; i, lobus olfactorius ; i', olfactory •. , f> j_' 



pedicle ;;L-,i:erv.olfactorius; ;, epiphysis ; ^l^WS traceS of gyratlOnS, 

 TO, nerv. oculo-motorius ; tr, uerv. trige- and when they are prO- 

 miniis ; v, nerv. vagus. • i i -ji i . i • 



vided with lateral vent- 

 ricles, tubercles representing cor'pora striata may be ob- 

 served. The olfactory pedicles take their origin from the 

 side of the hemispheres, and are frequently hollow, and 

 if so, their cavity communicates with the ventricle of the 

 hemisphere. The optic lobes are generally smaller than the 



Fig. 43. — Brain of Carcharias. (After Owen.) 



ac, Nerv. acusticus ; h, corpus restiforme ; c, 

 cerebellum ; d, lobus opticus ; c, hypo- 



