,THE GLOBE OF TEE EYE. 929 



In the normal condition, the transparency of the cornea results from the 

 equal refraction of the connective tissue fibrillae, and the chondrigenous substance 

 surrounding them. 



The external layer is only the conjunctival epithelium spread over the anterior 

 face of the cornea. This epithelium in the Horse measures '014 mm. in thick- 

 ness ; it is stratified, flattened on its surface, but cyhndrical below, where it 

 rests on the middle layer, and from which it is not separated — as in many other 

 species of animals — by a proper limitary membrane. 



The m?ier layer is a portion of the membrane of the aqueous humour. It is 

 composed of : 1. A hmitary membrane — the posterior elastic lamina (or nmnbrane 

 of Descemet) — •002 mm. in thickness, which becomes slightly fibrous at the peri- 

 phery of the cornea, where it forms, in passing on to the iris, the pectinated 

 ligament. 2. A lining of polygonal (flat) epithelial cells {endothelium), which 

 are provided with a large nucleus. 



(Some authorities give five layers to the cornea, the first being the corneal 

 epitheUum, and the second the anterior elastic lamina, or Boivmaii's membrane. 

 The third consists of a very elastic tissue, perfectly structureless, and possessing 

 a remarkable tendency to curl up ; but boiling, or the action of acids, does not 

 render it opaque, as with the other layers. Very fine fibres— ;^&roe arcuatoe — pass 

 obliquely between it and the next layer — the substantia propria, or cornea proper 

 — consisting of a large number of strata with branched fusiform cells. The fourth 

 layer is the posterior elastic lamina ; and the fifth layer consists of the epithelial 

 cells already mentioned.) 



Vessels. — The cornea has Httle vascularity in the adult. The vessels form 

 loops around its borders, but in the foetus they advance to near its centre. 



Nerves. — These were discovered by Schlemm. They penetrate by the peri- 

 phery of the cornea, and form a network on its surface. According to Kiihne, 

 Hoyer, Conheim, and Poncet, the ultimate nerve-ramifications pass into the 

 epithelium on the anterioir surface, and arrive between the most superficial cells. 



(The nerves of the eyeball perforate the sclerotic in the region of the 

 optic nerve, and run between it and the choroid anteriorly. During their 

 course, they give off branches provided with ganglion cells, to the choroid, and, 

 finally, they form a ring-like plexus in the ciliary region, termed the orbiculus 

 gangliosus ciliaris. From this plexus branches go to the ciHary muscle, the iris, 

 and the cornea. The nerves for the cornea pass first into the sclerotic, and form 

 a plexus round the margin of the cornea — the plexus cmtmlaris — from which 

 branches run into the conjunctiva and the cornea. The fibres entering the 

 substance of the cornea lose their white substance, and run on as naked axis- 

 cylinders to form networks in the different layers.) 



B. Musculo-Vascular Membranes. 

 3. The Choroid Coat (Figs. 502, c ; 503). 



(Preparation. — If the cornea has not yet been removed, it and the sclerotic may now be 

 dissected away from the choroid or second tunic. The connections between them are closeet 

 at the circumference of the iris, and at the entrance of the optic and ciliary nerves and arteries. 

 Fine blnnt-pointed scissors are necessary. A small portion of the sclerotic, near its anterior 

 circumference, is pinched up and clipped off, the edge of the incision is raised, the circum- 

 ference of the sclerotic divided, and that tunic removed piecemeal ; a gentle pressure with the 

 edge of the knife will remove it from its attachments around tlie circumference of the iris. 

 This dissection is best conducted under water. The ciliary nerves and long ciliary arteries 

 will be seen passing forward, between the sclerotic and choroid, to the iris.) 



