140 SPECIES NOT 



impressions proceed to the brain, and there produce 

 consciousness. What, therefore, has the fact, (if 

 shewn to be true,) of a sensitive nerve being 

 made sensitive to light to do with the organ of 

 vision? The eye-ball is an optical instrument, 

 as well as a light-seeing organ; and the whole 

 mechanism is so beautiful, that one almost trembles 

 with delight in recognizing there the unmistakable 

 evidence of Creative Wisdom and Design. 



That retina, of which every one must have 

 heard something, what is it? Let the general 

 reader imagine a vast collection of structures, 

 each in itself complete, and only capable of re- 

 ceiving a single pencil of light after it has passed 

 through a series of lenses. These structures or 

 areas, as they have been described by Professor 

 Goodsir, from the recollection of whose invaluable 

 oral instructions I am now quoting, are each 

 separately formed of 1st., what is called the limiting 

 or most internal membrane; 2nd., fibres of optic 

 nerve; 3rd., layers of poly-clonic nerve-cells; 4th., 

 an internal granular layer ; 5th., an external gra- 

 nular layer; 6th., a beautiful rod and cone, forming 

 part of what is termed Jacob's membrane. Now 

 of areas so formed, the retina contains a number 

 beyond all calculation, each being acted upon by 

 a single ray. All the operations of vision are 

 mechanically produced by this wonderful apparatus 

 when the ray reaches the optic nerve, except one, 

 and that the most important, namely, consciousness. 

 The optic nerve conveys the optical operations to 

 the brain, and there produces in fact, vision. 



