84 ZOOTOMY. 



latter are seen to form a continuous series, from the 

 myelocoele (v. 4), which is continued behind into the central 

 canal of the spinal cord, through the mesoccele (aq.s) to the 

 thalamocoele (v. 3). The latter is further seen to send a 

 small prolongation forwards into the otherwise solid pros- 

 encephalon, and another downwards and backwards into the 

 infundibulum. 



LII. Make out the chief structures in the eye, first 

 viewing from the outside,then removing successively 

 the sclerotic ( 224), choroid ( 225) and retina 

 ( 227) from the flattened dorsal surface. 



224. The cartilaginous sclerotic or outer covering of the 

 eye-ball : externally it passes into the transparent fibrous 

 cornea, which is the part exposed to view in the entire 

 animal : internally, the sclerotic is pierced by a small aperture 

 for the passage of the optic nerve, and is raised into a knob- 

 like prominence for articulation with the ophthalmic 

 peduncle ( 200). 



225. The black choroid lining the sclerotic: at the 

 junction of the latter with the cornea, it passes into the iris, 

 the coloured (yellow and black) part of the eye, seen from 

 the outside through the transparent cornea. The iris is 

 pierced by a central aperture, the pupil, the upper margin 

 of which is produced into a sort of curtain, divided at 

 its free edge into several processes. Between the iris 

 and the cornea is a space, the anterior chamber 

 of the eye, filled with a watery fluid, the aqueous 

 humour. 



226. The crystalline lens, a nearly globular transparent 

 body, situated immediately behind the iris. 



227. The retina, a delicate, greyish-white membrane, 

 lying immediately within the choroid, from which it is 

 readily detached except at the place of entrance of the optic 



