Metasoa Rana. 



325 



point towards, or rather have their free ends abutting 

 against, the latter. The nervous layer and the terminal 

 epithelial cells (named the rods and cones from their ap- 

 pearance) form the retina. Lastly the posterior chamber is 

 filled by a gelatinous substance, the vitreous humour, while 

 the anterior chamber similarly contains a more watery fluid 

 allied to lymph, the aqueous humour. A ray of light trans- 

 mitted through the cornea and aqueous humour is focussed 

 on the retina by means of the crystalline lens. The cur- 

 vature, and therefore the focussing power of the lens, can 



FIG. 181. ILLUSTRATION OF THE METHOD OF FOCUSSING RAYS OF LIGHT 

 ON THE RETINA. (Landois and Stirling.) 



A, point of origin of rays ; B, cornea; c, iris; D, lens; E, point of acutest 

 vision ; F, scierotic. 



be altered to suit rays coming from different distances by 

 means of the ciliary muscle. The iris is also provided with 

 radiating and circular muscle-fibres by means of which the 

 size of the pupil can be altered so as to allow of the entrance 

 of more or less light according to circumstances. The rays 

 pass through the vitreous humour and penetrating the retina 

 are reflected back from the choroid on the rods and cones. 

 The excitement produced there is carried by means of the 

 elements in the nervous layer of the retina to the optic nerve 

 itself, and by that means to the brain. 



