STRUCTURE OF THE CORNEA. 595 



The membrane of Demours or Descemet (fig. 400, 4) (posterior 

 eiasttc lamina, Bowman), not very closely united with the fibrous part 

 of the cornea, is transparent and glassy in 

 appearance. It is firm and structureless, but Fig. 402, 



very brittle and elastic ; and when shreds are 

 removed from it they tend to curl up with 

 the attached surface innermost. It appears 

 not to be affected by acids, by boiling in 

 water, or by maceration in alkalies. In 

 thickness it varies from -3 ^W ^0 2 o'o^ ^f an 

 inch. It is lined with an epithelioid cover- pj„ ^^.^ _p^j^,j ^^ ^^^ Epi. 

 ing (fig. 400, 5), which resembles that on thelioid Laveu of the 

 serous membranes, consisting of a single Membrane of Demours. 



layer of flattened polygonal cells with dis- 

 tinct nuclei (fig. 402)."' At its circumference the membrane breaks up 

 into bundles of fibres, which are partly continued into the front of the 

 iris, forming the " pillars of the iris," and partly into the fore part of the 

 choroid and sclerotic coats. 



To these festoon-like processes passing between the iris and posterior 

 part of the cornea, at its junction with the sclerotic, and which are very 

 much more marked in the eyes of the sheep and the ox than in the 

 human eye, the name ligamcnfum pedinatum iridis was given by 

 Hueck. The processes in question are covered with epithelioid cells, 

 continued from Descemet's membrane, but these cells do not stretch 

 across the intervals between the processes, so that the cavity of the 

 aqueous chamber is prolonged into, and freely communicates with, 

 cavernous spaces * in the tissue between the processes (fig. 403, 3). A 

 similar, but rather larger space is found slightly anterior to these in the 

 substance of the sclerotic, close to its junction with the cornea. This, 

 which is elliptical in section, is known as the sinus circidaris iridis, 

 ov canal of Schlemm (fig. 403, 4), and is said to communicate, through 

 the other spaces, with the aqueous chamber of the eye, owing to the 

 open nature of the tissue in this situation, and the breaking up of the 

 membrane of Demours into distinct bundles, as just described. But, on 

 the other hand, the canal of Schlemm, and the other cavernous spaces 

 in its neighbourhood, are in communication with the veins of the 

 anterior part of the sclerotic, and therefore the aqueous chamber must 

 also through them communicate with the veins. In support of this, 

 it is stated that both the spaces and the veins become filled with 

 coloured fluid when this has been injected into the anterior chamber. 

 Why the blood does not find its way into the latter during life is not 

 fully understood, since no valves have as yet been discovered in the 

 veins which lead out from the spaces : the reason is, probably, that a 

 greater resistance is oflPered to its passage here than to its return by 

 the ordinary paths. 



The above statements are mainly derived from the account given by 

 Schwalbe ; but, according to Leber, the results obtained were due to a 

 diffusible colouring matter having been employed for filling the anterior 

 chamber : when a non-diffusible one is used it never penetrates into 

 the canal of Schlemm, which is simply a large circular terminal vein, or 

 a collection of two or three plexiforni veins uniting at frequent intervals 

 into one trunk. 



■* Larger iu many animals, wliere tliey are known as the spaces of Fontana. 



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