OPTIC NERVES. 



Fig. 427. 



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," 

 " Fa'tits a trompe" " Monops" " Kinence- 

 phale," eye. Sfc. 



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!834. Theannexed 

 wood-cut (Jig. 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- 

 pearances fou nd by Geoffrey 

 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, Q t - t 

 course, and relations, &c. manCydops. (From 

 The chiasma is contracted a dissection by the 

 in front, and from it, only writer.) 



a single optic nerve pro- a > a > tractus P l1 - 



j .1 v ., cus ot either side; 



ceeds; this passes directly b> chia c> sin >. 



forwards to the back of the g i e op tic nerve ; d, 

 eye-ball, where it penetrates sclerotic coat of the 

 the sclerotic and terminates eye perforated by the 

 in the retina. P tic nerve - 



Fig. 428 represents the encephalon, optic 

 nerve, and organ of vision in a kitten at the full 

 period of gestation, (the subject of the same 

 form of monstrosity,) which lately came into 

 the writer's possession : the preparation is pre- 

 served in the Museum of the Richmond Hos- 

 pital School, Dublin. In all essential particu- 

 lars this specimen bears the closest resemblance 

 to the human monsters of which the dissection 

 has just been described. 



The fundamental defect in these monstrous 



a 



777 

 Fig. 428. 



Brain and organ of vision of 

 a Cyclops kitten, at the full 

 period of gestation. Seen from 

 below. (Natural size.) 



a, organ of vision, single, 

 and of great dimensions ; b, b, 

 cerebral hemispheres seen from 

 below ; d, d, tubercula quadri- 

 gemina ; c, optic nerve, single, 

 and of great size. 



foetuses consists in the total absence of the 

 organ of smell, in consequence of which de- 

 ficiency the symmetrical organs at either side 

 become united in the middle line and actually 

 engrafted upon each other: the two eyes are 

 conjoined so as to form but a single organ of 

 vision, and the very same metamorphosis occurs 

 in the two orbits, the two optic foramina, the 

 two optic nerves, &c. 



That this is the rationale can scarcely admit 

 of doubt, since in some parts of the organs 

 the fusion remains incomplete ; thus two crys- 

 talline lenses still exist in the interior of the 

 solitary eye-ball : a double set of muscles with 

 their corresponding nerves are provided for the 

 globe of the eye ; and four eye-lids protect the 

 organ in front, causing the aperture of the lids 

 to assume a quadrangular form. 



General developement of the optic nerves in the 

 higher clauses of animals. 



Fish. In fish as a general rule these nerves 

 are highly developed, and exhibit a marked 

 preponderance in size when contrasted with 

 the corresponding nerves in many animals 

 holding a more exalted position upon the scale. 

 This may be explained by the nature of the 

 medium which the fish inhabits; for some of 

 the light incident on the surface of the water is 

 reflected, and another part, after penetrating the 

 water, becomes absorbed, in consequence of the 

 continual disturbance to which the transparency 

 of this fluid is subject ; so that fishes necessa- 

 rily require a greater developement of visual 

 apparatus than would suffice land animals for 

 an equal amount of vision. 



Birds. In birds the sense of sight exists in 

 great perfection, and the optic nerves exhibit 

 corresponding developement. 



Mammalia. In Mammalia the faculty of 

 vision ceases to preponderate, and accordingly 

 the proportions of the optic nerves in this class 

 are no longer excessive. 



Many facts in comparative anatomy war- 

 rant the conclusion that the senses of smell 

 and vision are at times supplemental to each 

 other; for example, the mole either possesses 

 no optic nerve, or if any such exist it is so 

 diminutive as to be most difficult of recogni- 

 tion, but the olfactory lobes of the brain and 

 the whole olfactory apparatus of the animal 

 exist in great perfection, and its subterranean 

 habits enable it to turn this latter function to 

 account, whilst a highly finished organ of 

 vision would have been an useless appendage. 

 In certain fish which frequent the mud or slimy 

 waters (as for instance the eels), the visual 

 apparatus is poorly developed, and the optic 



