408 



THE SKIN AND THE SENSE-ORGANS. 



belong-ing" to the true corneal substance appear to pass into the 

 posterior hyahne layer, although they cannot be traced further 

 through its substance. The structure of the membrane is, in con- 

 sequence of its transparency, unknown, though the above ob- 

 servation seems to point to a fibrillar origin. 



(5) The corneal endothelium is a single layer of polygonal 

 cells of 0'02 mm. diameter. The cells possess the power of altering 

 Fio-. 255. their shape when stimulated 



(Klebs). 



(6) The nerves of the 

 cornea are derived from the 

 ramus ophtJialmicn trigemini; 

 they pierce the sclerotic 

 coat in front of the scle- 

 rotic cartilage and then 

 course towards the cornea, 

 at the margin of which they 

 form a coarse network of 

 medullated fibres. From 

 this about thirty nerves pass 

 towards the cornea, which 

 they enter, and then very 

 quickly lose the main part 

 of their medullary sheaths. 

 According- to Wolff, a por- 

 tion of the nerves retain their medullary sheaths, or in some eases 

 appear to regain it after having lost it. 



The nerves passing from the plexus (nerves of the first order, 

 Klein) give off smaller branches, which for a short distance have a 

 serpentine or rectilinear course. By a few anastomoses they form 

 a loose plexus (nerves of the second order, Klein). After a longer 

 or shorter course they give off numerous lateral fibres, or termi- 

 nate in several such fibres arising at one point (nerves of the 

 third order, Klein). These are distinguished by their size, varying 

 only within small limits, and by the possession of more or less 

 regularly placed varicosities ; the clearer portions are longitu- 

 dinally striated as though made up of fi})rillae ; they have a 

 nearly rectilinear course, and, after a longer or shorter course, 

 turn into a direction which is at right angles to the former 

 one; lastly, they remain for long distances unbranched. These 

 nerves are connected one with another by cross fibres running at 



Preparation from cornea of Rana escukrita ; after 

 Klein. Hartnack's Syst. , Obj. X immers., Oc. 3. 

 Endothelial cells. 

 Nuclei of endothelial cells. 

 Nerves of third order in the tissue of the cornea 



propria. 

 Nerves of the fourth order. 



