THE EYE. 375 



nerve enter the eye, each independently, 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 colour, 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 greyish, and transparent, like the finest 

 tubules in the central organs, measuring on the average not 

 more than 00006 — 00008'" and a good many only 00002 — 

 0-0004"', whilst some, it is true, have a size of 0*001 — 00015'" 

 or even of 0*002" / . What chiefly distinguishes these from 

 other pale nerve-terminations, is the absence of nuclei in their 

 course, a somewhat greater refractive power, and the frequent 

 occurrence of varicosities, which two latter particulars would 

 seem to indicate, if not exactly a nerve-medulla as in the 

 common nerves, still the existence of partially semi-fluid and 

 perhaps fatty contents and assimilate 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 

 circumstance at present conclude that they do not exist. 1 

 At any rate, the retinal fibres are not composed solely of 

 nerve-medulla, for, if they are treated never so thoroughly 

 with ether, they always remain, smaller indeed, but more dis- 

 tinct and more opaque than previously. Fibres which have 

 been thus treated enlarge again in cold acetic acid, and dissolve 

 in alkalies, and consequently consist, perhaps without doubt, 

 chiefly of a nitrogenous substance. 



As respects the course of the nerve-fibres in the retina, this 

 much is certain, that they radiate on all sides from the colli- 

 culus nervi optici and constitute a continuous membranous 

 expansion, which extends as far as the or a serrata retina, and 

 presents any considerable interruption only in the situation of 

 the macula lutea. In this true nervous membrane the fibres 



1 [Here again, Professor Kblliker is at variance with Mr. Bowman, who gives 

 as one of the peculiar characteristics of the fibres of the optic nerve in the retina, 

 that they have lost " the tendency to fall into the varicose or beaded state ; in a 

 word, that the fibres of the nerve, in expanding into the retina, lose their white 

 substance, but retain the axis- or central fibre." — Eds.] 



