OPTIC NERVES. 



767 



" The size of the optic thalami is not in 

 general in direct proportion either to that of the 

 optic nerves or the acuteness of vision in 

 animals. 



"In most fishes the optic nerves are of great 

 size, and the perfection of vision is extreme, yet 

 in this class the optic thalami are absent. 



" In birds, some of which enjoy exquisite 

 powers of sight, the optic thalami are small. 



" In mammalia the optic nerves bear no fixed 

 proportion to the optic thalami ; for instance, 

 the horse, the ox, and the stag have larger optic 

 nerves than the human subject, and yet the 

 optic thalami in these animals are infinitely 

 smaller than in man." 



The foregoing arguments are not conclusive, 

 for if the want of direct proportion between the 

 optic thalami and optic nerves were a proof that 

 the optic nerves draw none of their roots from 

 the optic thalami, the very same principle would 

 deprive the tubercula quadrigemina likewise of 

 all claim to be considered a source of the 

 second pair; since in the Mole and some other 

 mammalia, already specified, the nerves in 

 question and the tubercula quadrigemina ac- 

 tually occur in inverse proportion to each other. 



In considering this question it should be 

 recollected that in the mammalia large optic 

 thalami are always found associated with small 

 tubercula quadrigemina, and vice versa ; and 

 the same remark applies to the optic thalami 

 and optic lobes of the other vertebrata ; thus in 

 birds, the optic thalami are small, but the optic 

 lobes are of large dimensions : in reptiles the 

 same proportions are apparent : in fish the 

 optic thalami disappear, but the optic lobes are 

 immense, and two inferior lobes (an additional 

 source of the optic nerves) are superadded. 

 These facts favour the presumption that the 

 optic nerves derive roots from the optic thalami ; 

 for if (as is most probable) the optic thalami 

 and the tubercula quadrigemina both afi'ord 

 origin to the optic nerves, they may be mutually 

 supplemental to each other ; and in that case 

 the reciprocal proportions of these eminences 

 will be a matter of no consequence, provided 

 only that their sum be proportional to the 

 nerves. 



In farther support of the opinion here advo- 



Fig. 415. 



cated, it should be borne in mind, that the 

 tractus opticus is clearly traceable to the surface 

 of the optic thalamus in the human adult sub- 

 ject, and the writer's experience has convinced 

 him that the same anatomical disposition is 

 very apparent in early fetal life (fig. 415). It 

 may be well to add that in all the orders of the 

 mammalia which he has had an opportunity of 

 examining;, the tractus opticus derives filaments 

 from the optic thalamus : in the Aor.se, although 

 a large proportion of the tractus can be traced 

 to the nates, its anterior fibres spring most 

 distinctly from the optic thalamus (jig. 416); 



Fig. 416. 



Human foetal brain. (From nature.) Lateral view. 

 About fourth month. 



a, a, optic nerves ; b, chiasma ; c, right tractus 

 opticus ; e, right optic thalamus ; f, mass ot the 

 tubercula quadrisremina ; g, g, cerebellum ; d, 

 posterior extremity of right cerebral hemisphere, 

 displaced to exhibit the origin of the optic nerve. 



Tubercula quadrigemina, together with portions of the 

 optic thalami and tractus opt id of a horse. (From 

 nature.) 

 a, a, nates ; b, b, testes ; c, c, optic thalami j 



d, d, tractus optici, springing partly from the nates, 



but deriving a great portion ot their roots from the 



optic thaldini, c, c. 



in the sheep precisely the same arrangement 

 exists : in the hare many filaments of the 

 tractus opticus originate in the optic thalamus : 

 and in carnivora and quadrumana a similar 

 disposition prevails. 



Recent microscopic discoveries in ovoloay 

 (if it be fair to argue from the developement of 

 the chick to the evolution of the human foetus) 

 tend to confirm the views here put forward. 

 Baer states that on the fourth day of incubation 

 the encephalon of the chick consists of several 

 cells, one of which corresponds to the third 

 ventricle, and another to the optic lobes, and 

 that these two cells are distinct from each other. 

 The first rudiment of the eye observable in the 

 chick occurs in the form of a vesicle which 

 shoots out from the parietes of the cell of the 

 third ventricle, and which becomes gradually 

 elongated and drawn out into a canal. On the 

 fourth day the eye represents a spherical cavity 

 communicating with the third ventricle by a 

 canal ; this canal is the rudimental optic nerve, 

 which becomes gradually solid, its cavity disap- 

 pearing after the sixth day. During the earlier 

 periods of growth there is no connection what- 

 ever between the optic nerves and the cell of 

 the optic lobes, but the nerves just specified 

 are from the very commencement in free com- 

 munication with the cell of the third ventricle, 

 and in the walls of that cell the optic thalami 

 are developed. 



