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 laminte elude observation.
When viewed on one side the nerve seems perfectly smooth, but on the opposite side numerous laminw may be distinctly obstrved ; these are parallel to each other, and runN.Iong 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 ncuri lema only, and altogether (their straight course excepted) this arrangement bears a striking similarity to the disposition of the lamime on the human cerebellum. A short section of the nerve divested of its neurilema is not unlike a closed book, the larni nw 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. ( El g. 426.) 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 autlio-,' rity that birds which at short distances possess remarkably quick and accurate powers of vision (niore especially when in such casos, 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 pow-, erful org,ans 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 scwi bility supposed to be thus conferred on the optic nerve ; but the more obvious effect of this organization is to increase the surface of the nervous material. A similar contrivance is at times resorted to in the nervous centres, as is exemplified in the cerebral hemispheres ; these masses in the higher and more intelligent animals being covered with large convolutions and deep sulci, while in the lower classes they are smooth and consequently possess a. super ficies of limited extent.
Optic nerve in that form of monstrosity known by the varied appellations of " Cyclops," " Fcetus a trompe," " Illonops," " Rincnce Oak," 4.c.'4-c.
The abnormal anatomy of the optic nerve is not in strictness comprised within the scope of the present article, but nevertheless a brief description of the above malformation will pro bably not be considered out of place.
A single eye placed in the middle line of the forehead, and in general a trunk or proboscis growing immediately above this solitary organ of vision, constitute the most striking apparent anomalies in monsters of this class. The writer is indebted to Dr. Johnson, Master of the Dublin Lying-in Hospital, for permis sion to dissect a specimen of this species of monstrosity in the human subject, and he has obtained through the kindness of Dr. William Wilson Campbell, (formerly assistant in the same establishment,) the particulars of another similar case exa- — mined by that gentleman in the year 1834..The annexed wood-cut (fig. 427) gives a faithful representation of the optic nerve in the prepara tions dissected by the writer and Dr. Campbell; and it agrees exactly with the ap I pearances found by Geoffroy Saint-Hilaire, in the cases which fell under his obser vation. The tractus optici present a very natural ap pearance, being two in num ber, and perfectly normal in their cerebral attachments, course, and relations, &c. The chiasma is contracted in front, and from it, only a single optic nerve pro 1 ceeds ; this passes directly forwards to the back of the eye-ball, where it penetrates ' the sclerotic and terminates I in the retina.