STRUCTURE OF THE EYE. ]53 



or point at which the rays meet falls exactly on the surface of 

 the retina at the bottom of the eye. The whole surface of the 

 posterior chamber of the eye being lined with a black^sub- 

 stauce, absorbs all the rays that would otherwise by being re- 

 flected cause an indistinctness in the image produced. The 

 objects placed within the range of vision are represented on the 

 retina in an inverted position ; and many physiologists have 

 supposed that it is the picture on the retina that is perceived 

 by the sensoriura ; hence they have puzzled themselves to ac- 

 count for the erect appearance of erect objects ; but there is no 

 reason whatever for imagining that the picture which we per- 

 ceive formed on the retina of an eye of which a portion of the 

 sclerotic and choroid coats have been cut out, is what in the 

 natural state of the organ is observed by the mind. All that 

 can be safely said on the subject is simply that the rays of 

 light reflected from objects are in the eye arranged so as to 

 produce on the delicate expansion of the retina an impression 

 which is conveyed by the optic nerve to the sensorium. 



The eyes of birds vary considerably in form, the convexity 

 of the cornea and lens being greater or less, and the proportions 

 of the other parts undergoing alterations. Thus in Owls, the 

 sclerotic zone is so large as to occupy more than a third of the 

 length of the eyeball, to which it gives somewhat of a cylin- 

 drical form. In the Pelicans, the sclerotic zone is narrow, and 

 the cornea much less convex than in Hawks and Owls. But the 

 manner in which the focus of distinct vision is adapted to dis- 

 tant and near objects is not well understood, and at all events 

 does not at present require our attention. The degree of con- 

 vexity of the cornea cannot have much effect, and does not 

 appear to be liable to much alteration in the same individual ; 

 but if the convexity of the lens may be increased or diminished, 

 or its distance from the retina altered, the eye can obviously be 

 thus adapted to various distances. 



There still remain to be noticed some clandular organs, 

 which are situated within the orbit. The lachrymal gland, 

 which is situated near the outer angle of the eye, is of a some- 

 what elliptical form, and small size. The fluid which it secretes 

 having moistened the surface of the cornea, is received into two 



