122 ZOOTOMY. 



176. A nerve running alongside the hyoidean artery is the posterior 

 or hyomandibular division of the seventh nerve or portio dura. 

 (See 193.) 



XXXIII. Remove the layer of transparent integument covering 

 the exposed surface of the eye ; cut away the supraorbital 

 process of the frontal with bone forceps, and, if time permit, 

 dissect out the ocular muscles and their nerves, which have 

 the same essential disposition as in the skate (pp. 77, 78, 191 

 203) ; then remove the eye, dissect away the muscles, &c. still 

 attached to it, and divide it into inner and outer hemispheres by 

 an equatorial incision, passing midway between the pupil and 

 the entrance of the optic nerve. Note the following : 



177. The cornea, iris, lens, sclerotic, retina, and aqueous and 

 vitreous humours, have the usual relations (pp. 84, 85, 224 228). 



178. The choroid, consisting of three distinct layers ; a black pig- 

 mentary layer next the retina, a shining silvery layer next the sclerotic 

 and between these a red vascular layer, which undergoes a great 

 thickening around the entrance of the optic nerve, forming a sort of 

 annular cushion ; this thickening is the so-called choroid gland. 



179. A slit in the retina extending from the blind spot or entrance of 

 the optic nerve, along the posterior (outer) side of the eyeball to the 

 ora serrata or outer boundary of the-retina ; through it extend a deli- 

 cate grey fold of the choroid, the processus falciformis, which ends 

 against the posterior side of the lens in a pyriform enlargement, the 

 campanula Halleri. 



XXXIV. The fifth nerve may now conveniently be dissected 

 ( X 93) > afterwards remove the skin in the neighbourhood of 

 the nostrils, and observe 



1 80. The small nasal sac, communicating with the exterior by the 

 two apertures already seen ( 86), but having >no connection with the 

 mouth: its wall is formed by the delicate, plaited Schneiderian 

 membrane. 



XXXV. Carefully break away the roof of the auditory capsule so 

 as to expose the organ of hearing ; or, better, remove the 

 entire auditory capsule, fasten it out firmly under water in a 

 dissecting-dish and dissect it from the inner or cranial side. 

 Note 



1 8 1. The irregular system of cavities excavated in the bone and 

 cartilage of the auditory capsule for the membranous labyrinth ; the 

 separation of these cavities from the brain cavity by membrane only. 



182. The membranous labyrinth (Fig. 33), consisting of the ovoidal 



