774 



OPTIC NERVES. 



rally throughout the animal series ; on the con- 

 trary, the chiasma usually conforms to the type 

 prevalent in the class to which the animal 

 belongs, without evincing in its conformation 

 much regard to the relative directions of the 

 optic axes ; and examples are not unfrequent in 

 which the anatomy of the optic nerves is at 

 variance with what the relative directions of the 

 optic axes would require theoretically. 



Thus in the pleuronectes fish, (Jig. 409,) the 

 optic axes are so directed that the two retinae may 

 be inferred to have a certain amount of identity, 

 and nevertheless, in such of them as have been 

 examined by the writer, the optic nerves are 

 severally derived from opposite sides of the 

 brain, and cross each other without forming a 

 chiasma ; or in other words, retinae evincing 

 mutual identity are supplied by optic nerves 

 which have no identity of origin ; the type pre- 

 valent in osseous fish being preserved, without 

 respect to the directions of the optic axes. In 

 many of the cetacea, the direction of the 

 optic axes is such that the retinae can have no 

 identity, and nevertheless a perfect chiasma, 

 such as occurs in other mammalia, exists in 

 these animals. And in the owl the eyes look 

 more directly forwards than those of most other 

 birds, from which it may be presumed that the 

 amount of mutual identity in the two retinae is 

 much greater in them than in those birds whose 

 eyes have a lateral aspect ; but nevertheless 

 the structure of the chiasma in the owl appears 

 in nothing different from that which prevails in 

 birds whose optic axes have a strictly lateral 

 direction. 



But further, when the optic axes are very 

 divergent, as in some quadrupeds, any object 

 which can be depicted upon both retinae simul- 

 taneously, will throw its images on the outer 

 parts of the two retinae, and in order to explain 



A 



single vision under such circumstances, the 

 outer parts of the two retinae should therefore 

 be reciprocally identical ; but these parts are 

 formed by the outer filaments of the optic 

 nerves, which come respectively from the cor- 

 responding sides of the brain, so that here, 

 reciprocally identical parts of the two retina, 

 instead of having a common origin at one and 

 the same side of the brain (as they ought in 

 order to suit theoretical views), are derived seve- 

 rally from opposite sides of the organ, and con- 

 sequently have no identity of origin (jig. 423). 

 These considerations are sufficient to falsify the 

 explanation of single vision put forward above, 

 and the writer is of opinion that hitherto the 

 exact use of the chiasma has not been disco- 

 vered. (See VISION.) 



Some remarkable varieties of optic nerves.^ 

 Optic nerves in certain cephalopods. The 

 loligo or calamary exhibits in its optic nerves 

 a singularly beautiful arrangement, which the 

 physiologist cannot but contemplate with the 

 greatest interest, as it presents the most perfect 

 decussation of nervous filaments hitherto dis- 

 covered. The loligo, in common with some of 

 the allied families of the cephalopoda, possesses 

 an extremely perfect organ of vision ; so elabo- 

 rate is the mechanism of the eye that it has 

 attracted a considerable share of attention from 

 comparative anatomists ; and the developement 

 of the optic nerve bears proportion to the per- 

 fection of the other parts of the visual apparatus. 

 The nervous system of the loligo conforms 

 to the cyclogangliate type, and from each late- 

 ral surface of the supracesophageal ganglion, 

 (fig. 424,) one of the optic nerves comes off. 

 After pursuing a short course outwards the 

 nerve swells into a large ganglion (the optic) ; 

 this body is oval in shape, and of enormous 

 dimensions ; it exceeds considerably the volume 

 of the supracesophageal ganglion. One surface 

 of the optic ganglion is directed towards the 

 eye, and emits an immense number of filaments, 

 which spring chiefly from its edges, and which, 



Fig. 424. 



Diagram to shew how in eyes with divergent axes, 



the images of every object placed so as to be seen by 



both eyes simultaneously fall on tfie outer parts of the 



two retinas. (After Mutter.) 



a-, x', y, y, axes of the eyes diverging ; a, b, c, 



objects seen by both eyes ; a', a", parts ot the two 



retinae on which the object a is depicted ; b', b", 



parts of the two retinae on which the object b is 



depicted ; c', c", parts of the two retinae ou which 



the object c is depicted. 



Organ of vision, together with the optic nerve and 

 supra-aesophageal ganglion in the loligo, much mag- 

 nified. (From a dissection by the writer.) 



a, supra -oesophageal ganglion; b, optic nerve; 

 C, optic ganglion ; d, d, one series of filaments 

 springing from the edge of the ganglion ; e, e, 

 second series of filaments derived from the op- 

 posite edge of the ganglion, and in this view visible 

 near their termination only ; f, f, points at which 

 the filaments of the two series decussate ; g, ball 

 of the eye. 



