ORGANS OF THE SENSES. 509 



those of the neighbouring planes at an angle, and seldom 

 communicating with them. The arrangement and size of 

 these tubes can be shown by driving mercury, or coloured 

 size, or air into a small puncture made in the cornea. They 

 may also be shown under a high power by moistening a thin 

 section of a dried cornea, and opening it out by needles." 

 In addition to the above it may be remarked, that the spaces 

 may be seen without injection, or any other preparation even 

 in perfectly fresh eyes. 



In vertical sections of the cornea, after the application of 

 acetic acid, the nucleated fibres in its outer and denser por- 

 tion are seen to follow the curved form of the cornea itself, 

 while in its inner and softer part they are variously disposed ; 

 horizontal sections of the cornea, even taken from the surface, 

 exhibit a curved and interlaced arrangement of the fibres ; 

 fibres also nucleated pass from the surface of the cornea 

 deeply into its substance: the use of these is doubtless to 

 assist in preserving its convexity. (See Plate LXVIL fig. 1.) 



The posterior elastic lamina is the third layer of the 

 cornea: it is a perfectly transparent membrane of appreci- 

 able thickness, so that it may be readily recognised with 

 the unaided sight, and is but slightly attached to the cornea 

 proper. 



It is usually described as structureless, and in most cases 

 it certainly is so, but in the human eye it frequently exhibits 

 peculiar markings, portrayed in Plate LXVII. Jigs. 11, 12. 

 These, however, would appear to proceed from definite in- 

 equalities of the surface rather than from any distinct fibrous 

 or cellular tissue ; nevertheless, the appearances observed are 

 remarkable and worthy of record. 



This lamina preserves its transparency even in boiling water 

 and acetic acid ; and a further peculiarity is, that although it 

 may be torn in any direction, it is so hard that it can be bitten 

 through, only with difficulty. It extends to the margin of 

 the cornea only, where it comes into connection with certain 

 elastic fibres to be described hereafter. 



The epithelium of the aqueous humour is the fourth layer 



ss 2 



