ANIMAL MECHANISM. 115 



by accident, or a want of skill, the operator suffer this 

 humor to escape, in any of his operations, the globe at 

 once diminishes in size, and all hope of the restoration of 

 a diseased eye is lost. The small mistake of pricking 

 the sack containing the vitreous humor, will decide 

 whether the patient is to live in never ending night. 



FIG. 8. 

 Explanation of Figure 8. 



One dotted line, indicates, in 

 this diagram, the aqueous hu- 

 mor ; another the iris, and a 

 third the lens, and the fourth 

 the vitreous humor. Let it be 

 remembered that all the space 

 between the back side of the lens 

 and optic nerve, is filled com- 

 pletely, with the glairy, vitreous 

 humor, the third fluid, and in- 

 most of the eye. 



OPTIC NERVE. 



Any person possessing an ordinary share of curiosity, 

 can examine the optic nerve, or nerve of 'sight, at leis- 

 ure, in slaughter houses, fish markets, and in fowls. In 

 the human eye, perhaps, to be clearly understood, out- 

 side the eye, as it extends to meet the brain, it is like a 

 cotton cord, larger than a wheat straw, of a rnealy white- 

 ness, not far from three quarters of an inch in length. 

 Arising from the substance of the brain, it traverses the 

 bony canal till it reaches the back of the eye-ball ; as 

 soon as it arrives in contact, as it were, it is suddenly 

 divided into innumerable filaments, which wend their 

 way into the globe, through very minute holes. From a 

 fanciful resemblance to a sieve, this spot on the scleroti- 

 ca, is called the cribriform plate. When the threads 

 have emerged within, they assume another form, by ex- 

 panding into a web, constituting a third or inmost box. 

 Some believe the nerve is spread on a thin, unseen mem- 

 brane, in the form of a highly organized nervous paste. 

 Here, on this pulp, having considerable range of surface, 

 is the sole seat of vision. A vulgar opinion presupposes 

 pome exceedingly acute nervous point, the exquisite 



