744 SPECIAL HISTOLOGY. 



diminish in size, under repeated divisions, till they are reduced to fine 

 filaments of scarcely 0-0004 of a line in diameter, Avhich, in isolated 

 cells, terminate in torn ends. In every case in Avhich I have noticed 

 these nerve-cells distinctly in situ, their processes were given off towards 

 the exterior, and afterwards in their further course, without entering 

 the granular layer, appeared to he curved, in order to ramify in the 

 gray nervous layer itself. The nuclei of these nerve-cells, which behave 

 towards reagents like those of the cerebrum, measure 0-003-0-005 of a 

 line, and usually present a very distinct nucleolus. 



4. On the inner aspect of the layer in question we find the expansion 

 of the optic nerve [d). This nerve, after quitting the cJiiasma (concern- 

 ing which vid. p. 390) and till it reaches the eye, presents the same 

 conditions as a common nerve ; its dark-bordered fibres, 0-0005 — 0-002 

 of a line in diameter, much disposed to become varicose, and between 

 which, according to Hassall, nerve-cells would also seem to occur, but 

 which I have not yet noticed, form polygonal bundles, 0-048-0-064 of a 

 line thick, surrounded by a nurilemma of the usual kind. When the 

 optic nerve reached the eye, its sheath is lost in the sclerotic, which 

 tunic is perforated for the entrance of the nerve by a funnel-shaped 

 opening, the narrower part being inward ; and the internal 7ieurilemma 

 also ceases on a level with the inner surface of the same tunic, where it 

 may be artificially displayed, as a cribriform lamella {lamina crihrosa of 

 authors), so that the fibres of the optic nerve enter the eye, each inde- 

 pendently, without their sheaths of connective tissue. Within the canal 

 of the sclerotic, and as far as the slight eminence, the colliculus nervi 

 optici, visible on the inner surface of the retina opposite its point of 

 entrance, the optic nerve retains its white color, and continues to pre- 

 sent dark-bordered tubules ; but, from that point onwards, its elements, 

 in Man and in many animals, become perfectly clear, yellowish or gray- 

 ish, and transparent, like the finest tubules in the central organs, mea- 

 suring on the average not more than 0-0006-0-0008 and a good many 

 only 0-0002-0-0004 of a line, whilst some, it is true, have a size of 

 0-004-0-0015 or even of 0-002 of a line. What chiefly distinguishes 

 these from other pale nerve-terminations, is the absence of nude} in 

 their course, a somewhat greater refractive power, and the frequent oc- 

 currence of varicosities, which two latter particulars would seem to in- 

 dicate, if not exactly a nerve-medulla as in the common nerves, still the 

 existence of partially semi-fluid and perhaps fatty contents, and assimi- 

 late the nerve-fibres of the retina to the most delicate elements of the 

 cerebrum. I .have not yet been able to demonstrate axis-fibres and 

 sheaths in the fibres of the retina, although I would not from that cir- 

 cumstance at present conclude that they do not exist.* At any rate 



* [Here again, Professor KoP.iker is at variance with Mr. Bowman, who gives as one of 

 the peculiar characteristics of the fibres of the oj^tic nerve in the retina, that they have lost 



