CHAP. XVII.] CHAMBERS OF THE EYE. 37 



a narrow circle of hyaloid membrane behind, as well as by the 

 ciliary processes which slope towards the iris, and thus limit the 

 lateral dimensions of the chamber. It is very easy to imagine the 

 existence of a lining membrane to this cavity of the aqueous humor, 

 such as would form a closed sac, and answer to the serous struc- 

 tures; and a membrane of the aqueous humor has accordingly been 

 described by several anatomists. But the most careful observation 

 fails to detect any such serous sac, though the posterior epithelium 

 of the cornea (p. 21) closely resembles that of serous surfaces. No 

 epithelium exists in front of the iris, and certainly none is present 

 on the anterior surface of the lens : this we can aver from repeated 

 examination. On the posterior surface of the iris, however, there 

 seems to be a pigmentary membrane. 



Of the Optic Nerves, and their Central Connexions. The second 

 pair of nerves is devoted to the sense of sight, and on that account 

 has received the name of Optic Nerves. 



The marked manner in which these nerves terminate in the 

 retinae, the constant relation in size between them and the organ of 

 vision, the atrophy which they suffer when the visual apparatus has 

 been destroyed, the impairment or loss of vision which follows a 

 morbid state of them, place it beyond all question that they are 

 the proper conductors of visual impressions to the sensorium. 



The optic nerves form a most extensive connexion with the brain 

 through the optic tracts. The optic tracts are two flattened bands 

 of nervous matter, which proceed from the pos- Fig . 125 . 

 terior and superior surface of the mesocephale (the 

 region of the quadrigeminal tubercles) forwards 

 along the inferior surface of each crus cerebri, 

 and after passing in a curved course (concave in- 

 wards) along the base of the brain, unite in front 

 of the tuber cinereum and rnamillary bodies, and 

 form a very intimate junction, which is called 

 the chiasma or commissure of the optic nerves. 



From this chiasma the optic nerves spring, and 



,. ., P j , .-i -i , Plan of the optic nerves 



diverge as they pass forwards into the orbits O n a smaii scale, showing 



. T 1,1 , { mi i. "u their divergence from the 



through the optic foramina. This point may be cWMma,,Snd their June- 

 looked upon, therefore, as their origin. To under- tSfSfJfS^SSKl 

 stand, however, more exactly their relation to the th 

 brain, it will be necessary to trace the connexions of the optic tracts, 

 and to inquire into the structure of the chiasma. 



In tracing each optic tract back from the chiasma, we find that it 

 first forms a pretty close connexion with the locus perforatus on the 



