398 THE CORNEA, BY ALEXANDER ROLLETT. 



of the cornea, Henle * and Schweigger-Seidel f may be con- 

 sulted. 



With crossed Nicol's prisms the transversely divided bands 

 appear dark in all azimuths, whilst the longitudinally and 

 obliquely divided bands appear alternately clear and dark. f 

 The optic axes thus coincide with the direction of the fibres. 

 The cornea as a whole, when examined as far as possible 

 free from folds, and with its natural curvature preserved, 

 gives, with crossed Nicol's prisms, a dark cross, || which does 

 not alter its position on rotating the membrane around an axis 

 passing perpendicularly to the surface of its vertex (axis of 

 the eye). 



This indicates that there is a preponderance of the fibrous 

 bands or fasciculi running* in the meridianal direction over 

 those passing in all other directions. If this be correct, the 

 following explanation of the cross may be given. In each of 

 the doubly refracting meridians one plane of polarisation passes 

 through the axis of the cornea (optic axis), whilst the other 

 plane of polarisation is vertical to the latter. Between the 

 crossed Nicol's prisms all those meridians must appear dark 

 whose planes of polarisation coincide with those of the ana- 

 lysers and polarisers, whilst the intervening meridians become 

 brighter in proportion as they are more remote from the dark 

 meridians.^ 



We must now consider more closely the layer of the cornea 

 that lies immediately beneath the external epithelium. Its 

 relative thickness in regard to the other layers is shown be- 

 tween b and V (fig. 377). 



* Eingeweidelehre, p. 595, fig. 454. 



f LOG. cit., p. 309, figs. 1 and 3. 



$ His, loc. cit., p. 28, et seq. 



Compare Boeck, Taf. ii., fig. 1, and Mtiller, Zeitschrift fur rationelle 

 Medicin, 3 Reihe, Band x., p. 173. 



|| Brewster, Philosophical Transactions, 1816, p. 315. Valentin, Unter- 

 suchungen der Pflanzen und Tfiiergewebe in polarisirten Lichte, (Researches 

 on vegetable and animal tissues in polarised light,) p. 270. Leipzig, 1861. 



"IT See v. Lang, Ueber das Kreuz das gewisse organische Kb'rper in 

 polarisirten Lichte zeigen, etc., (On the cross that certain organic bodies 

 exhibit in polarised light,) Poggendorff's Annakn, Band cxxiii. 



