276 



COMPARATIVE ANATOMY 



layer, the sclerotic, which is also surrounded by a lymph-space. 

 The latter passes internally into the sheath of the optic nerve, 

 which is continuous with the dura mater, and externally into the 

 cornea, the outer surface of which is covered over by an epithelial 



layer continuous with the epi- 

 derm the conjunctiva. The 

 sclerotic and cornea together 

 form a firm outer support for 

 the eye, and thus, together with 

 the gelatinous mass of the vit- 

 reous humour, guarantee the 

 rigidity necessary for the physio- 

 logical activity of the nerve 

 end-apparatus. Between the 

 cornea and iris there is a large 

 lymph-space, the aqueous cham- 

 ber, its contained fluid being 

 called the aqueous humour : ex- 

 tending around the chamber is 

 a venous plexus, which is bathed 

 by the aqueous humour. 



The relative development of 

 the eye is affected by the ex- 

 ternal conditions, 1 and is in 

 general proportional to the 

 rapidity of the movements per- 

 formed by the animal concerned 

 and to the relative development 

 of the mid-brain. 



In all Vertebrates the eye- 



FIG. 203. DIAGRAM OF A HORIZONTAL 

 SECTION THROUGH THE LEFT HUMAN 

 EYE, seen from above. 



C, ciliary process ; Ch, choroicl, with 

 its lamina fusca (Lf) and vascular 

 layer (OS) ; Cj, conjunctiva ; Co, 

 cornea ; CP, canal of Petit ; CS, 

 venous plexus (canal of Schlemm) ; 

 Cv, vitreous chamber ; Fo, yellow 

 spot (fovea centralis) ; H, hyaloid 

 membrane ; HK, so-called posterior 

 chamber : Ir, iris ; L, lens ; Lc, 

 ciliary ligament ; MD, posterior 

 elastic lamina (membrane of Des- 

 cemet) ; MF, blind-spot ; Op, optic 



nerve ; OS, sheath of optic nerve ; ball is surrounded by a mem- 



, retina ; PE, pigment epithelium b ranouS) saC -like investment 



(the periorbita), which arises in 

 the region of the optic foramen 



of retina ; Sc, sclerotic ; VK, aqueous 

 chamber; Z, Zonula ciliaris (zone of 

 Zinn). 



and radiates outwards towards 

 the skin, its distinctness being less marked the more completely 

 the orbit is surrounded by skeletal parts. As in most Vertebrates 

 the cavity of the orbit is more or less continuous with the temporal 

 and oral cavities, the orbital sac has a close relation to the muscles 

 of the jaws, certain portions of which may invade it and take on 



1 The adaptive modifications of the eye are very varied amongst Vertebrates. 

 It may reach a very high degree of perfection ; or may, on the other hand, 

 undergo more or less degeneration in those animals which live in caves or burrows, 

 or in the deep sea (e.g. Fishes Amblyopsis spelseus, Troglichthys, Typhlo- 

 gobius ; Amphibians Proteus, Spelerpes maculicauda, Typhlotriton, Typhlo- 

 molge, Gymnophiona ; Reptiles Typhlops vermicularis, Khineura floridana ; 

 Mammals Notoryctes typhlops, Talpa). In the Cetacean Platanista gangetica 

 the eyes are extremely minute. The reduction of the eyes in many deep-sea 

 Fishes may be compensated for by the special development of tactile organs. (Cf. 

 also under Cyclostomes, p. 278). 



