From the statementsof Tiedemann, it appears that the corpus geniculatum externum is much more tardy of developement in the fcetus than the optic nerve itself, for the eminence in ques tion becomes only for the first time apparent about the sixth month of fcetal life: again Serres affirms that both corpora geniculata appear so late as the sixth month of uterine existence; and according to the joint testimony of these two authorities, the corpom geniculata are de veloped in the course of the tractusopticus, and superadded to the rudimental optic nerve.
The late appearance of the corpora geniculata in the embryo,and the manner of their develope ment, would seem to assimilate these tubercles to the ganglions found in the course of certain nerves of special sense in many animals, and which are perhaps destined to exalt the sensi bility of the nerves in which they occur. The optic ganglions of the loligo ( fig. 424, c), and the olfactory ganglions of many fishes, afford good examples of the nervous masses to which allusion is here made.
Tuber cinercum : its relation to the optic nerves.
The same difficulties'uniformly encountered in all attempts to determine the particular functions of individual parts of the brain pre vail in the case of the tuber cinereum : some physiologists maintained that the optic nerves derive a great number of filaments from that body, and that the nerves are considerably in creased in dimensions by this addition : Gall, for instance, was of this opinion, and gave a rather exaggerated representation of the enlarge ment supposed to arise out of this reinforcement to the nerves.
The optic nerves in man may dnubtless draw some of their roots from the tuber eine reum, but there is an absolute certainty that the; body in question has other and probably mores important functions than any connected witl the origin of the nerves of vision. Pathologiea and experimental observations upon the subjec are still a desideratum, but in the absence more direct evidence the anatomy of the mole brain is calculated to throw some light up the enquiry. In the mole the optic nerv either wholly absent or if present, it is m rudimental; nevertheless, the tuber cinereu of enormous dimensions • it extends forwar the olfactory lobes, and' so far back war nearly to reach the pons. In this animal, tl fore, there is an inverse proportion ap between the optic nerve and tuber eine a fact little favourable to the hypothesis a cated by Gall.
Of the chiasma 9f the optic nerves. word chiasma (from the Greek xicsubcor, d satio,) means in strictness a decussati crossing at acute angles, like the leg letter X ;* and, for convenience-sake, t expression (with a like latitude of appli will be here employed to designate the responding structure in the lower animals.
The organization of the human chiasina abundantly exercised the ingenuity of a mists, who seem to have encountered difficulty in their attempts to trace the ne filaments through it ; and consequently, withstanding all the attention bestowed the subject, opinions the most conflict prevailed upon the true nature of the In question. In no other instance is junction between two corresponding opposite sides known to occur. Su anomaly affords strong presumptive eviden the existence of some unusual properties' nerves thus united ; and for these reasons the physiology of the chiasma is invested with u ncom mon in terest: The existence of a chiasma is not general throughout the animal series, and even when present it exhibits much diversity of appearance and structure in different classes. A brief ex position of some of its more striking varieties, In animals, will probably constitute the best introduction to the study of the chiasma in man.
invertebrata.—In the invertebrate classes nothing like a chiasma has been demonstrated, nor has any mutual crossing of the special optic nerves been proved to exist. The nerves which are furnished to the compound eyes of insects and crustaceans pass in a direct course to their destination ; the same remark applies to the nerves from which the lens-eyes of insecta, arachnida, crustacea, and mollusca, derive their sensibility ; and it may be presumed that the nerves which supply the simple eye-dots of annelida and other inferior animals are similarly circumstanced.
Osseous fish.—In osseous fish the optic nerves generally cross each other at an acute angle, in such manner that the nerve which comes from the right side of the brain goes , distinctly to the left eye, and vice versa : at the r point of decussation the nerves lie one over the other ; they are usually flattened at this spot, and closely joined together, but this junction is effected by means of cellular or fibrous ad hesions only, as no intermingling of the nervous 1 filaments takes place, and the nerves themselves can be isolated without injury to their proper structure (fig. 407).