OPTIC NERVES. 



411. 



765 



what parts of the mammal's brain 

 are analogous to the optic lobes of 

 the lower classes. 



The tubercula quadrigemina in 

 man and the mammalia are iden- 

 tical with the optic lobes of the 

 lower vertebrata ; they occur as 

 four small tubercles arranged in 



pairs, 



of which the anterior are 



Brain of a Turtle. ( From nature.) Lateral view. 



called the nates, and the posterior 

 the testes. Tn some of the class, 

 as for example, Kuminantia, Soli- 

 aa, optic nerves; b, chiasma ; c, optic lobes ; d, tractus options, pe da, and Rodentia, the nates are of 

 connecting the right optic lobe to the chiasma. ' . ' 



larger dimensions than the testes ; in 



immense in birds of prey, and much smaller in others, as for instance, Carmvora, the testes 

 other birds not equally remarkable for perfec- predominate in size over the nates, and in 

 tion of si u ht (Ji- a . 412). Man and Quadrumana the two pairs are nearly 



equal. 

 -Fig. 412. fig. 413. 



A B C 



Brain of an Eagle. ( From nature.) Seen from below. 



a, a, optic nerves ; c, chiasma, of immense size ; 

 b, b, optic lobes of large dimensions, placed at the 

 inferior and lateral aspect of the encephalon. 



The situation of these bodies in the brain of 

 the bird, so different from their position in 

 reptiles and fish, created at one period some 

 doubts as to their true analogies ; but Serres 

 has shown that during the early stages of deve- 

 lopement the optic lobes occupy precisely the 

 same position in the encephalon of the chick 

 which they hold permanently in the brain of 

 the reptile and fish, and he has thereby divested 

 this subject of much of its obscurity. Thus 

 before the tenth day of incubation the optic 

 lobes of the chick are placed between the cere- 

 bellum and the cerebral hemispheres, and are 

 then best seen at the dorsal aspect of the brain : 

 but after this epoch the hemispheres and cere- 

 bellum approach each other at the expense of 

 the optic lobes the hemispheres extending 

 backwards, and the cerebellum inclining for- 

 wards. By this double movement the optic 

 lobes are soon overlapped behind, separated 

 from each other, and at length pushed down- 

 wards and outwards to their permanent si- 

 tuation (fg. 413). 



In Man the optic nerves derive some roots 

 from the tubercula quadrigemina. 



In birds, reptiles, and Ash, the optic lobes 

 constitute the principal sources of the optic 

 nerves, and therefore in any attempt to ascer- 

 tain the true origin of the second pair in man, 

 a necessary preliminary will be to determine 



Brain of a chick. (After Serres. ) At three different 

 stages of incubation. 



A, at sixth day; B, at tenth day; C, at four- 

 teenth day. 



A, a, a, optic lobes ; b, rudimental cerebellum ; 

 c, c, cerebral hemispheres. 



B, a, a, optic lobes separated from each other in 

 front, and here slightly depressed ; b, cerebellum 

 inclining upwards and forwards between optic lobes ; 

 c, c, cerebral hemispheres growing backwards so as 

 to overlap the optic lobes. 



C, a, a, optic lobes still farther separated from 

 each other and depressed towards base of brain ; 

 ft, cerebellum growing upwards between the optic 

 lobes ; c, c, cerebral hemispheres carried backwards 

 so as to come nearly into contact with the cere- 

 bellum. Reference tojiy. 412 will shew the brain 

 of the bird in its full-giown condition. 



The tubercles in question have but little 

 apparent similarity to the optic lobes of the 

 lower Vertebrala : they occur as four eminences, 

 while the optic lobes of birds, reptiles, and 

 fish, are but two in number : they are of dimi- 

 nutive size; the optic lobes of birds, reptiles, 

 and fish are of large dimensions in proportion 

 to the brain : they are solid; the optic lobes of 

 birds, reptiles, and fish are hollow : and in 

 Man and most Mammalia they are covered 

 upon the upper surface by the cerebral hemi- 

 spheres, while the optic lobes in reptiles and 

 fish are not so covered. Such obvious dissimi- 

 larity tended materially to obscure the real 

 nature of the tubercula quadrigemina, but a 

 careful study of the developement of these 

 bodies in the fetal brain led anatomists at 

 length to discover their true analogies ; and 

 the researches of Tiedemann and Serres have 

 chiefly contributed to establish the following 

 particulars. 



" In the earlier stages of uterine life the tuber- 

 cula quadrigemina of Man and Mammalia 



