In considering this question it should be recollected that in the mainmalia 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 isource 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 1 nd the tubercula quadrigemina both afford rigin to the optic nerves, they may be mutually upplemental to each other ; and in that case e reciprocal proportions of these eminences vill be a matter of no consequence, provided nly that their sum be proportional to the ierves.
In farther support of the opinion here advo 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 fcetal life (fig. 415). lt 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 hiorse, although a large proportion of the tractus can be traced to the nates, its anterior fibres spring most distinctly from the optic thalamus (fig. 416); 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 nvology (if it be fair to argue from the developement of the chipk to the evolution of the human fcetus) 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 distinctirom each other. The first rudiment of the eye observable in the chick °cons in the forrn 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 vvith 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 grovvth 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 nf that cell tbe optic thalami are developed.
'The evidence which pathology has afforded upon this question must be considered unsa tisfactory in the extreme ; for, on the one hand, well authenticated cases are recorded in which vision remained perfect although the optic thalamus was extensively diseased, and Gall and Spurzheim have ohserved atrophy of the optic nerves to reach the nates without affecting the optic thalamus : while, on the other, Cru veilhier has seen the corpus geniculatuin ex ternum involved in the wasting of the optic nerve, and Magendie and Desmoulins, from their own researches and experiments as well as from those of Ncethig and Scemmerring, infer that after long-continued blindness the atrophy of the _optic nerve in man sometimes affects the optic thalamus.
The following is a summary of facts favour able to the supposition that the optic nerves in man derive roots from the optic thalami.
1. The human tractus opticus admits of being distinctly traced to the optic thalamus, both in the fcetal condition and subsequently to birth.
2. In many, if not all, of the mammalia, the optic nerves in the clearest manner derive roots from the optic thalami.
3. The optic nerve of the chick first appears as an offset from the third ventricle, and the optic thalami are developed in the walls of that ventricle.
4. The inverse proportion known to subsist between the tubercula quadrigemina and the optic thalami in mammalia, and also between the optic lobes and the optic thalami in birds, reptiles, and fish, may probably be considered corroborative facts.
Corpora geniculata : their relation to the optic nerves.
That there is an intimate physiological rela tion between the optic nerves and the corpom geniculata can scarcely admit of doubt, for the principal band of the human tractus opticus is, in every instance, traceable to the corpus geniculatum extemum, and may be seen actually incorporated with that tubercle ; and a similar connection between the lesser band of the tractus and the corpus geniculatum intemum is also, for the most part, discoverable : more over, in various orders of the mammalia a portion of the tractus opticus emanates most distinctly from the corpus geniculatum in ternu m ; and in quadrumana, carnivora,rodentia, &c., this has been frequently. verified by the writer.