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neurilemma is of the more delicate kind, to which Virchow 

 has given the name neuroglia. Its fibres have two principal 

 directions, one transverse to that of the nerve-bundles and 

 therefore parallel to those of the lamina cribrosa, the other 

 vertical to the front of the lamina and free surface of the 

 nerve-disc. These last fibres correspond to the radial con- 

 nective-tissue-fibres in the retina. 



This sheath and its septal prolongations consist of common 

 connective-tissue fibrillated bundles with corpuscles inter- 

 spersed. 



In front, this division of the sheath blends with the lamina 

 cribrosa and with the inner third of the sclerotic. 



The outer sheath is continuous posteriorly with the dura 

 mater, so that coloured fluids injected into the space between 

 the two sheaths soon find their way backwards into the 

 cranial cavity. In front, the outer sheath is continued into 

 the outer two-thirds of the sclerotic. The outer and inner 

 tubes are loosely connected by a very open areolar tissue, 

 composed of curling fibres and coarser bundles, with large 

 fusiform nucleated corpuscles imbedded in them. These give 

 the interstitial or areolar spaces the appearance of having an 

 epithelial lining. They play an important role in neuritis 

 and in the evolutions of morbid growths. 



The space between the sheaths has been lately described 

 as a lymphatic cavity. In severe injuries of the head, blood 

 is sometimes extravasated into it. 



The Pecten. — In describing the sclero-choroidal foramen, I 

 mentioned the occasional excess of pigment here giving rise to a 

 dark circle around the nerve-disc, visible in the living eye. The 

 presence of pigment in the disc itself is still more exceptional; 

 I do not allude to pathological formations but to congenital 

 conditions. In some mammalia, however, the neuroglia of 

 the optic disc is always pigmented — granule pigment dotted 

 along the central fibres and in corpuscles. In these eyes the 

 pigment corpuscles are often most numei'ous along the vessels 

 and in the loose tissue in the sheath close to the globe ; but 

 the greatest development of pigment here is in birds, from 

 whose optic nerve entrance a large plaited membrane stands 

 forwards into the vitreous toAvards the back of the lens, with 

 a forward and downward inclination. The upper edge lies 

 below the level of the fovea centralis. Its general figure is 

 four-sided, but it varies somewhat, as do also its size and the 

 number of plaits. 



It is a vascular sheet, consisting of large vessels and a very 

 close capillary net overlaid Avith a pigmented epithelium, 

 resembling that of the choroid. Its base blends with the 



