GENERAL STRUCTURE OF THE RETINA. 223 



nerve fibres of very various diameter are mingled together, the 

 finer ones preponderating. As the nerve perforates the sclero- 

 tic at the so-called lamina cribrosa, all the nerve fibres, except 

 in a few instances that will hereafter be mentioned, lose their 

 medullary sheath. The diminution in the diameter of the 

 nerve, thus occasioned, is rather sudden, and is accompanied 

 also, according to Lowig,* by the internal connective tissue of 

 the nerves becoming continuous with that of the sclerotic and 

 choroid. What now remains of the nerve fibres is the 

 extremely delicate axis-cylinder, completely deprived of its 

 medullary sheath. These, enclosing the arteria and vena cen- 

 tralis, and always invested by a certain quantity of connective 

 tissue, pass through the choroid, and as they radiate outward 

 in all directions, bound the shallow crater of excavation of the 

 optic diskf into the plane of the internal surface of the retina, 

 where they form the optic-fibre layer situated immediately 

 external to the membrana limitans interna, and the thickness 

 of which gradually diminishes towards the ora serrata, so that 

 at this latter line only isolated fibres or small fasciculi of fibres 

 are demonstrable. At the yellow spot of the retina the layer 

 of nerve fibres, considered as a continuous layer, suffers an in- 

 terruption. The rest of the connective tissue of the optic 

 nerve passes into the substance of the supporting fibres of the 

 retina.^: 



The nature of the nerve fibres forming the layer in question 

 may best be examined under the microscope in the perfectly 

 fresh state by placing portions of the retina taken from the 

 still warm bulb in vitreous humour, with the internal surface 

 looking upwards. Under these circumstances we may some- 

 times, in the vicinity of the ora serrata, where the optic-nerve 

 fibres run separately, obtain very well-marked specimens, pro- 



* Studien des Physiolog. Institute zu Breslau, herausgegeben von Reichert, 

 1858, p. 125. 



t H. Miiller treats of the so-called physiological excavation of the 

 entrance of the optic nerve in Grafe's Archiv fur Ophthalmoloyie, Band 

 iii. , Abtheil. ii. , p. 86. L. Mauthner refers in a more extended manner 

 to the recent literature upon the subject in his Lehrbuch der Ophthalmm- 

 copie, 1868, p. 252. 



Klebs, in Virchow's Archiv, Band xix., p. 321, Taf. vii. 



