250 ZOOTOMY. 



optic nerve, ends in a tendon which passes through the 

 sheath of the bursalis, then proceeds downwards and back- 

 wards, curves round the postero-ventral margin of the eye- 

 ball, and finally passes forwards to be inserted into the edge 

 of the nictitating membrane. 



312. The sclerotic, cornea, iris, and pupil have the 

 same relations as in other Vertebrata (see p. 84, 224, 225). 



313. The sclerotic plates, small flat bones, arranged 

 in an imbricating manner round the sclerotic just external 

 to its junction with the cornea. 



LI. Divide the eye, by an equatorial incision, into 

 inner and outer hemispheres : note : 



314. The pecten, a black plaited structure, in the 

 antero-ventral region of the inner hemisphere, having a 

 curved free edge, and an attached edge which extends from 

 the entrance of the optic nerve downwards and forwards, 

 parallel to the course of the pyramidalis ( 311) : it consists 

 of a fold of the choroid, projecting through a fissure in the 

 retina. It should be removed and examined under a low 

 power of the microscope. 



315. The choroid, retina, lens, and aqueous and 

 vitreous humours have the same relations as in the 

 other Vertebrata (see pp. 84, 85, 224 228). 



316. In the outer hemisphere, the choroid processes, 

 radiating folds, developed in the outer part of the choroid, 

 between the outer edge of the iris and the ora serrata 

 or free border of the retina. 



LII. Cut away the walls of the external auditory 

 meatus ( 128), and make out: 



317. The tympanic membrane, a thin sheet of 

 fibrous tissue stretched across the bottom of the external 

 meatus. 



318. By carefully removing the tympanic membrane, the 



