xm 



PHYLUM CHORDATA 



155 



the diencephalon. From this opens in front a median prosocoele, 

 which gives off a pair of paracodes (para.) extending into the two 

 lateral portions of the prosencephalon. 



From the anterior enlargements of the olfactory bulbs already 

 mentioned spring numerous fibres which constitute the first pair 

 of cerebral nerves and enter the olfactory capsules. Between the 

 two olfactory lobes two small nerves, the terminal or pre-olfactory , 

 arise from the prosencephalon : they are the nerves of ordinary 

 sensation for the interior of the olfactory sacs. From the optic 

 chiasma the two optic nerves (Figs. 831, 833, 834, II) run outwards 

 through the optic foramina into the orbits, each perforating the 

 sclerotic of the corresponding eye and terminating in the retina. 

 The third, fourth, and sixth pairs of 

 nerves have the general origin and 

 distribution which have already been 

 described as universal in the Craniata 

 (p. 100). 



The trigeminal (Figs. 831, 833, 834, V) 

 arises in close relation to the facial. 

 As it passes into the orbit it swells 

 into a ganglion the Gasserian. Its 

 chief branches are three in number. 

 The first given off is the superficial 

 ophthalmic (Fig. 833, oph. V ; Fig. 834, 

 V op.), which runs forwards through 

 the orbit above the origin of the recti 

 muscles, and in very close relation 

 with the ophthalmic branch of the 

 facial. Anteriorly it breaks up into 

 branches distributed to the integument 

 of the dorsal surface of the snout. 1 

 The main trunk of the nerve then runs 

 forwards and outwards across the 

 floor of the orbit, and divides into 

 two branches, the maxillary and mandi- 

 bular, or second and third divisions of 

 the trigeminal. The former (mx. V) 

 supplies the skin of the ventral surface of the snout, the latter 

 (mnd. V) the skin and muscles of the lower jaw. 



Of the branches of the facial, the ophthalmic runs through the 



1 In most Elasmobranchs a nerve of considerable size the ophthalmicus 

 profundus (Fig. 788) arises from the dorsal and anterior part of the Gasserian 

 ganglion, and is usually regarded as a branch of the trigeminal. It runs 

 forwards over the posterior rectus muscle and under the superior rectus, and 

 perforates the pre-orbital process to end in the integument of the snout. 

 Among other branches it gives off ciliary branches to the iris : these are 

 joined by the ciliary branches of the oculomotor. An ophthalmicus profundus 

 is not present in Scyllium in the adult condition. 



melct. 



Fia. 832. Hemiscy Ilium. The 



brain viewed from the dorsal side, 

 the roofs of the various ventricles 

 removed so as to show the rela- 

 tions of the cavities (semi-dia- 

 grammatic), cer, dilatation from 

 which the epicoele is given off ; dia, 

 diacoele, pointing to the opening 

 leading into the infundibulum ; Her. 

 iter or mesocoele ; meta. metacoele ; 

 opt. optocoele ; para, paracoele ; 

 pros, prosocrele ; rh. rhinocoele. 



