620 



MANUAL OF HISTOLOGY. 



Rapidly decreasing in size, the nerve fibres quickly lose their medullary 

 sheath, and are then suddenly found, at a greater or less distance from 

 the border of the cornea, in the form of filaments, reduced in diameter to 

 0'0009 mm., which, under the action of certain reagents, are seen to be 

 varicose. The bundles of fibres run in a direction towards the centre and 

 anterior surface of the cornea, and in their course divide and subdivide 

 over and over again, forming by intercommunication a nervous plexus at 

 the nodal points, of which nuclei may be observed. At the same time, an 

 increase of the number of the fine fibrillae unmistakably takes place, 

 while, on the other hand, the perineurium is no longer to be seen. 



Of these nervous plexuses there are several, lying one over the other. 

 The most anterior, with its delicate bundles of fibres, was looked upon by 



earlier observers (His) as a 

 terminal network of the 

 nerves. From it, however 

 (fig. 577), bundles of fibres 

 are again given off, which 

 pierce the anterior surface of 

 the cornea (Hoyer, Cdhn- 

 heim), and, splitting up into 

 tassels, form that sub- 

 epithelial interlacement al- 

 ready mentioned ( 177), 

 whose perpendicularly as- 

 cending fibres terminate in 

 the epithelium (Cohnheim). 



At its border this last in- 

 terlacement receives other 

 twigs, which enter the cornea 

 with the minute vessels of that part, and, ascending more or less abruptly, 

 takes part in the formation of the plexus, and extend also as far as the 

 anterior surface. 



Besides these sensory nervous interlacements, the cornea possesses 

 deeper plexus-like ramifications of nerves. KuJme maintained, many 

 years ago, that in the frog the termination of the varicose end filaments 

 of these in the epithelial cells might be seen ; but this has not been since 

 confirmed (Koelliker, Engelmann, Hoyer). Here also the primitive fibrillse 

 probably terminate, in part at least, with free ends. They are to be found 

 but rarely in the posterior layers of the cornea, in larger numbers in the 

 middle, and tolerably abundant in the anterior portions. A plexus, seated 

 here under the lamina elasfica anterior, has been described by Hoyer. 



310. 



We turn now to the uvea or tunica vascvlosa, with its various consti- 

 tuents, mentioned above, and find the arrangement of parts much more 

 complex than in the structures just dealt with. 



The choroid is usually described as consisting of an external fibi'ous 

 coat, and an internal single layer of pigmentary flattened epithelium, 

 which latter belongs, properly speaking, to the retina, as we learn from 

 the history of its development. In fig. 578 we have the latter figured 

 again ; but it has been already described, at greater length, in 150. As 

 we shall have to refer to these cells again in speaking of the retina, we 

 shall, for the present, say no more about them. 



Fi. 577. Vertical section of the cornea of a rabbit, a, ft, 

 epithelium; d, a nervous twig; e. /, sub-epithelial distri- 

 bution of fine varicose nerve nbrillse ; /, distribution and 

 termination in epithelium. 



