OPTIC NERVES. 



Plaited optic nerve. 



The optic nerve in some animals exhibits a 

 peculiar plaited appearance ; this condition 

 occurs in greatest perfection in certain fishes, 

 and in many birds a somewhat similar organi- 

 zation may be detected, though not at all so 

 perfectly as in the fish tribes. 



When the plaiting prevails in perfection, the 

 nerve consists essentially of a thin membrane 

 folded on itself exactly like a closed fan or a 

 plaited frill ; but the arrangement referred to is 

 not at first sight apparent, particularly if the 

 nerve be inspected in that part of its course only 

 which is external to the cranial cavity, for there 

 the neurilema is so thick, dense, and tough, 

 that a correct estimate of the disposition of the 

 nervous structure cannot be formed until this 

 investment has been removed. 



Deprived of its neurilema, the nerve seems to 

 consist of a number of parallel laminae placed 

 in juxta-position; on closer examination these 

 laminae turn out to be continuous with each 

 other at their edges, and by a little care the 

 nerve can be unfolded into a membrane of 

 which the breadth is proportional to the num- 

 ber and depth of the original laminae. 



When the plaited condition is perfect, it pre- 

 vails along the entire length of the optic nerve, 

 becoming manifest at its cerebral attachment, 

 and continuing to the eye-ball; even the retina 

 seems to participate in the same disposition, 

 as folds or plaits are observable on the surface 

 of that nervous expansion ; and in certain fish 

 the optic lobes themselves present a similar 

 organization, for the walls of the cavities which 

 these tubercles contain are in some instances 

 covered with laminae. 



The plaiting must be considered an essential 

 attribute of the nervous substance and totally 

 independent of the neurilematous investment, 

 for this disposition of the nervous material 

 occurs occasionally in the optic lobes and re- 

 tina, structures which are devoid of neurilema. 

 (Fig. 425.) 



Fig. 425. 



Plaited optic nerve of a 

 Mullet. (From nature.} 



a, optic nerve deprived 

 of neurilema, exhibiting 

 the plaited disposition ; 

 6, sclerotic coat of the 

 eye through which the 

 nerve is passing ; c, 

 retina, in which the 

 nerve terminates. 



Laminated optic nerve. 



In certain birds the optic nerve is laminated 

 and bears a close resemblance to the plaited 

 condition just described. 



In these birds a careful examination of the 

 nerve is required for the discovery of its true 

 texture, for the neurilema (endowed with un- 

 common strength) adheres so firmly to the 

 nervous structure that without a careful dis- 

 section its laminae elude observation. 



When viewed on one side the nerve seems 

 perfectly smooth, but on the opposite side 

 numerous laminae may be distinctly observed ; 

 these are parallel to each other, and run along 



the nerve in the longitudinal direction : they 

 are of considerable depth and variable thick- 

 ness; they are applied closely to each other, 

 being separated by thin processes of neuri- 

 lema only, and altogether (their straight course 

 excepted) this arrangement bears a striking 

 similarity to the disposition of the laminae on 

 the human cerebellum. A short section of the 

 nerve divested of its neurilema is not unlike a 

 closed book, the laminae representing the leaves, 

 and the opposite smooth suiface of the nerve 

 bearing a resemblance to the back and cover 

 of the book. The laminated optic nerve of the 

 bird does not admit of being unfolded into a 

 flat membrane, a proceeding which can be 

 accomplished with care in the plaited optic 

 nerve of the fish : and in birds the laminated 

 structure occurs in that part of the nerve only 

 which is in front of the chiasma; in its cerebral 

 extremity no such organization prevails. (Fig. 

 426.) 



Fig. 426. 



Laminated optic nerve of an 

 Eagle. (From nature.) 



~\ 

 a 



a, chiasma ; b, optic nerve 

 divested of neurilema and 

 exhibiting lamina 1 on one 

 surface ; c. sclerotic coat. 



Desmoulins, who has paid the greatest at- 

 tention to these varieties, inferred as the result 

 of his extensive researches, that the plaited 

 arrangement of the optic nerve in fish, and 

 its laminated condition in birds bears pro- 

 portion to the perfection of vision in the ani- 

 mals under consideration; for those birds 

 which are endowed with the most powerful 

 piercing sight possess the laminated structure 

 at a maximum of developement. For example, 

 birds of prey, such as the eagle, the falcon, 

 and the kite, evince an acuteness of this faculty 

 truly surprising; from heights in the atmos- 

 phere, at which they are themselves almost 

 invisible, they discover their prey on the ground 

 and pounce on it with the most unerring cer- 

 tainty, whilst at the trifling distance of a few 

 yards other animals recognise such objects with 

 difficulty: now birds of this class afford the 

 most perfect specimens of the laminated optic 

 nerve. It is further stated on the same autho- 

 rity that birds which at short distances possess 

 remarkably quick and accurate powers of 

 vision (more especially when in such cases 

 this faculty is exercised in media of different 

 degrees of density) are also provided with 

 laminated optic nerves, and that most fish in 

 which the plaited optic nerves occur are of pre- 

 dacious habits, and consequently require posv- 

 erful organs of vision just as the birds of like 

 propensities. 



The writer has examined the optic nerves in 

 the stork, kingfisher, eagle, &c., and in them, 

 as well as in all the fishes which have fallen 

 within his reach, his dissections have amply 

 verified the anatomical descriptions given by 

 Desmoulins. 



It is difficult to explain the superior sensi- 

 bility supposed to be thus conferred on the 



