ZOOLOGY AND BOTANY, MIOEOSCOPY, ETC. 29 



Distribution and Termination of Nerves in the Cornea. * — 

 Opinions have differed widely as to the actual mode of termination of 

 tlie corneal nerves, whether singly or by fasciculi in the corneal cells, 

 or by reticulations surrounding them. These and kindred questions 

 have been investigated by Professor G. V. Ciaccio. He has studied 

 animals from all the Vertebrate classes except fishes, and has chiefly 

 employed chloride of gold to render the nervous elements visible. His 

 results are summed up as follows : — 



1. The nerves of the cornea are of different kinds and have 

 different functions, viz. (a) sensitive, some to light and some not, and 

 (b) trophic, regulating the nutrition of the tissue. 



2. They form a plexus, the " circumferential nervous plexus", at 

 the circumference of the cornea before entering it ; this consists 

 partly of medullated, partly of non-medullated fibres. 



3. This plexus sends out branches and twigs of different sizes in 

 various quantities, which enter the cornea, divide and subdivide there 

 and form a plexus, the " primary or principal nervous plexus," which 

 traverses its entire breadth ; in the rabbit, mouse, rat, and bat it lies 

 chiefly near the anterior face ; in lizards, tortoises, frogs, and tritons 

 it is near the middle of its thickness ; in birds it is mostly contained 

 in its anterior portion. 



4. Other plexuses exist in this organ, more or less derived from or 

 dependent on this chief one ; they are termed secondary or accessory ; 

 they sometimes lie above, sometimes below the chief one. In the 

 frog this plexus lies below the latter, and close to Descemet's mem- 

 brane ; in the mouse, it lies above, close to the anterior face of the 

 cornea and thus constitutes the " subbasal plexus " of Hoyer and 

 others. 



5. The principal plexus gives off a large number of small branches, 

 sometimes accompanied by ultimate fibres ; they are termed " per- 

 forating branches " ; they break up first below the epithelium, each 

 into a tuft of fibrils, which form between themselves the " subepi- 

 thelial plexus," of greater or less closeness, and differently arranged 

 in different animals. In the mouse and rat, and perhaps the bat, it 

 has a concentric arrangement, but the centre does not correspond to 

 that of the cornea. 



6. From different places in the subepithelial plexus fibrils go off 

 and enter the epithelium, dividing and anastomosing, and thus forming 

 in it a very delicate reticulation, probably broken off here and there, 

 (the intra-epithelial rete or plexus of modern authors) ; the fibres 

 terminate either in small button-like dilatations or simply below 

 the outermost cells of the epithelium, which form a delicate mem- 

 brane interposed between these endings and the exterior. 



7. The various plexuses and networks thus formed are not to be 

 considered as so many distinct units but as so many compound systems, 

 each of them being made up of as many parts as there are nerves 

 entering into its constitution. Thus, by their distribution over the 

 cornea, the nerves form just so many anatomically and physio- 

 logically distinct regions as there are trunks and branches of nerves. 



* Mem. Accad. Sci. 1st. Bologna, ii. (1881) 24 pp. (2 pis.)— Sep. repr. 



