OPTIC NERVES. 



775 



after making their way to the eye-ball, and 

 perforating the sclerotic, terminate in the retina. 

 These filaments are arranged at their origin in 

 a double linear series, each series streaming 

 forth from one of the opposite margins of the 

 ganglion. After becoming free of the ganglion 

 the filaments of each series approach those of 

 the other series, and a most perfect and regular 

 interlacement of the two sets ensues ; so com- 

 pletely do they decussate that either half of the 

 retina is formed by filaments which spring 

 from the opposite edge of the ganglion ; those 

 filaments which shoot out from the anterior 

 margin of that body terminating in, and actu- 

 ally forming, the posterior moiety of the retina, 

 and vice versa. 



The reciprocal interlacement of these nervous 

 filaments reminds one of the manner in which 

 the two series of threads in a weaver's loom 

 (technically called the woof and the warp), cross 

 each other; this illustration is exact, for the 

 interlacement of the threads in the one case is 

 not more perfect or regular than that of the 

 nervous filaments in the other. 



The writer is indebted to his colleague, Dr. 

 Power, for a knowledge of this curious arrange- 

 ment, which was discovered by that gentleman 

 while inspecting a preparation of the nervous 

 system in the loligo made by the writer, and 

 which still exists in the Museum of the .Rich- 

 mond Hospital School, Dublin. 



Subsequent dissections have proved satisfac- 

 torily that the decussation in question occurs 

 invariably in the loligo, and that in the octopus, 

 which exhibits close affinity to the loligo, the 

 same arrangement prevails. In the sepia 

 officinalis the optic ganglion is reniform, and 

 its filaments come off in a double series, as in 

 the loligo ; the majority of the filaments decus- 

 sate after the same manner as that above de- 

 scribed in the other two species, but some of the 

 extreme filaments of each series pass in a direct 

 course to the retina, without exhibiting any de- 

 cussation. 



The peculiar disposition of the optic filaments 

 in these cephalopods was unnoticed until the 

 publication of Dr. Power's paper on the sub- 

 ject in the " Dublin Journal of Medical Science," 

 an omission the more surprising, as accurate 

 descriptions of the eye in this class of animals 

 have appeared from some of the ablest ana- 

 tomists. 



Swammerdam long ago described and de- 

 lineated the optic ganglion of the sepia offici- 

 nalis, but without making any mention of the 

 intercrossing of the filaments which emanate 

 from that body. Cuvier, in his memoir on the 

 Mollusca, describes the eye of the poulpe 

 (octopus) with great accuracy, but makes no 

 allusion to the decussation in question ; and in 

 the most recent descriptions of the cephalopods 

 the same omission occurs. 



The above discovery seemed at first likely 

 to afford an explanation of the problem so 

 difficult of solution, namely, the well-known 

 fact, that objects appear erect to a spectator 

 although their images on the retina are inverted ; 

 and for some highly ingenious observations 

 tending to elucidate this obscure question, the 



reader is referred to the paper by Dr. Power, 

 already quoted ; it may be stated, however, that 

 many difficulties remain to be removed before 

 such an explanation can be received, for 



1. The filaments of the optic nerve of the 

 loligo have not as yet been traced back to the 

 central masses of the nervous system, and there 

 can therefore be no certainty that they preserve 

 in the first part of their course the same relative 

 positions which they are known to maintain in 

 the interval between the optic ganglion and 

 point of decussation. 



2. In vision the object is known to be wholly 

 reversed in the image on the retina ; for ex- 

 ample, that which is above in the object is 

 below in the image, and vice versa ; and that 

 which is to the right in the object is to the left 

 in the image, and vice versa ; now, although 

 the decussation in question might possibly 

 explain the correct appreciation of an object 

 whose image is reversed in one particular 

 direction (say from above downwards), it never 

 can fully explain the correct impression made 

 by an object of which the image is reversed iu 

 all directions simultaneously. 



3. In order that this explanation may apply 

 to human vision, the same interlacement of 

 filaments must first be demonstrated in the optic 

 nerve of man, a task which has not as yet been 

 accomplished. 



Optic nerves of the compound eyes of Insects. 



These nerves are excessively large and appear 

 proportional to the size of the organs of vision 

 in the Insect. 



Each nerve on arriving at the eye swells out 

 into a bulb, which is convex and varies con- 

 siderably in dimensions in different species ; 

 this bulb in general represents a segment of a 

 sphere, and from its surface nervous filaments 

 in immense numbers arise and diverge like the 

 radii of a circle. Several thousand of these 

 filaments have been counted in a single nerve ; 

 each of them is connected by its distal extremity 

 to the apex of a small conical transparent body 

 interposed between the filament and the cor- 

 responding facet of the cornea, but the most 

 striking peculiarity of the nerve is found in the 

 perfect isolation of each of these filaments by 

 the interposition of pigment. 



The pigment is variable in colour, being 

 sometimes light, at others dark; it may be 

 nearly black, dark violet, dark blue, purple, 

 brown, brownish yellow, light yellow, or green, 

 and sometimes several layers of different colours 

 lie one over the other; the pigment extends 

 from the bulb of the nerve to the cornea ; it sur- 

 rounds each filament and separates it com- 

 pletely from adjacent filaments. 



The effect of this disposition is to isolate the 

 rays of light incident on each filament, and 

 to prevent the transmission of all rays except 

 such as fall in the direction of the axis of the 

 filaments ; since all oblique rays must of neces- 

 sity impinge upon the colouring matter and be 

 in consequence absorbed. 



The reader is referred to the article INSECTA 

 for further details concerning the optic nerve in 

 these animals. 



