768 



OPTIC NERVES. 



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 observed atrophy of the 

 optic nerves to reach the nates without affecting 

 the optic thalamus : while, on the other, Cru- 

 veilhier has seen the corpus geniculatum 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 foetal 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 corpora 

 geniculata can scarcely admit of doubt, for the 

 principal band of the human tractus opticus 

 is, in every instance, traceable to the corpus 

 geniculatum externum, and may be seen actually 

 incorporated with that tubercle; and a similar 

 connection between the lesser band of the 

 tractus and the corpus geniculatum internum 

 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- 

 ternum; and inquadrumana, carnivora,rodentia, 

 &c., this has been frequently verified by the 

 writer. 



From the statements of Tiedemann, it appears 

 that the corpus geniculatum externum is much 

 more tardy of developement in the foetus than 

 the optic nerve itself, for the eminence in ques- 

 tion becomes only for the first time apparent 

 about the sixth month of foetal 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 corpora geniculata are de- 

 veloped in the course of the tractus opticus, 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 cinereum : 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 doubtless 

 draw some of their roots from the tuber cine- 

 reum, but there is an absolute certainty that the 

 body in question has other and probably more 

 important functions than any connected with 

 the origin of the nerves of vision. Pathological 

 and experimental observations upon the subject 

 are still a desideratum, but in the absence of 

 more direct evidence the anatomy of the mole's 

 brain is calculated to throw some light upon 

 the enquiry. In the mole the optic nerve is 

 either wholly absent, or if present, it is merely 

 rudimental; nevertheless, the tuber cinereum is 

 of enormous dimensions; it extends forwards to 

 the olfactory lobes, and so far backwards as 

 nearly to reach the pons. In this animal, there- 

 fore, there is an inverse proportion apparent 

 between the optic nerve and tuber cinereum, 

 a fact little favourable to the hypothesis advo- 

 cated by Gall. 



Of the chiasma of the optic nerves. The 

 word chiasma (from the Greek ^icio-pos, decus- 

 satio,) means in strictness a clecussation, or 

 crossing- at acute angles, like the legs of the 

 letter X;* and, for convenience-sake, the same 

 expression (with a like latitude of application) 

 will be here employed to designate the cor- 

 responding structure in the lower animals. 



The organization of the human chiasma has 

 abundantly exercised the ingenuity of anato- 

 mists, who seem to have encountered great 

 difficulty in their attempts to trace the nervous 

 filaments through it ; and consequently, not- 

 withstanding all the attention bestowed upon 

 the subject, opinions the most conflicting have 

 prevailed upon the true nature of the structure 

 in question. In no other instance is a similar 

 junction between two corresponding nerves of 

 opposite sides known to occur. Such an 

 anomaly affords strong presumptive evidence of 

 the existence of some unusual properties in the 



* In Human Anatomy the term is used (without 

 perhaps sufficient regard to its etymology) to ex- 

 press the nervous mass in which the two optic 

 nerves are conjoined. 



