VOL. LXXXIX.] PHILOSOPHICAL TRANSACTIONS. 435 



the nerve had the same structure in its whole coui se. For this purpose, transverse 

 sections were examined in different parts of the nerve, near the brain towards the 

 middle, and nearer the eye: of these experiments the following are the results. 

 In all the sections, the nerve appeared to be made up of the same substances ; but 

 the size and number of the opaque parts differed very much. They have been 

 stated, near the eye, to be 600; about the middle of the nerve, they were 150; 

 and, near the brain, between the origin and union of the 2 nerves, they were only 

 about 40. As they became larger, they were less regular in their shape, and had 

 less of a circular form ; nor were they uniform, some appearing very large, with 1 

 or 2 smaller placed between them. 



After having succeeded in this examination of the nerve transversely, an at- 

 tempt was made to investigate its structure in a longitudinal direction. To do this, 

 a portion of the nervous pulp had its coat, formed by the dura mater, along with a 

 thin vascular membrane which lines it, carefully removed for about an inch in 

 length ; the external surface of the pulp was then examined with a magnifying 

 glass; the structure was evidently fasciculated, but the fasciculi did not run parallel 

 to each other; they seemed to unite together and separate again, in such a manner 

 that any one of them could not be traced for half an inch in length, without being 

 lost in the neighbouring part. When thin sections were examined in the field of 

 the microscope, they put on the same appearance: this was equally the case, whe- 

 ther the part examined was near the centre or circumference of the nerve. The 

 fasciculi were largest in that part of the nerve near the brain, and smallest towards 

 the eye. Great pains were taken to ascertain whether the fasciculi were made up of 

 continued fibres, or of small parts unconnected, which, from their position, gave 

 that appearance; but every observation that was made was in proof of their being 

 continued fibres. 



From these experiments, the internal structure of the optic nerve appears to be 

 made up in the following manner: At its origin from the brain, it consists of 30 

 or 40 fasciculi or bundles of extremely small opaque pulpy fibres, the interstices 

 between which are filled with a transparent jelly. As the nerve goes farther from 

 the brain, the fasciculi form smaller ones of different sizes. This is not done by 

 a regular subdivision, but by a few fibres going off laterally from several large 

 fasciculi, and being united, forming a smaller one: some of the fasciculi so 

 formed, which are very small, unite again into one. In this way, the fasciculi 

 gradually diminish in size, and increase in number, till they terminate in the 

 retina. Near the eye, where the fasciculi are most numerous, the substance of 

 the nerve has a considerable degree of transparency, from the number of trans- 

 parent interstices between them; but this is less the case nearer the brain, where 

 the interstices are fewer. In the optic nerve of the cat, the structure is the same 

 as in the horse; but, from the smallness of the parts, less fitted for investigation. 

 Near the eye, its internal substance is more transparent than the corresponding 

 part in the horse. 



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