768 
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 Nethig 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 in quadrumana, 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 secon only for the first time apparent 
about the sixth month of feetal 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. 
late appearance of the corpora geniculata 
in the embryo, and the manner of their develope- 
‘between the optic nerve and tuber ciner 
OPTIC NERVES. 
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, ¢), and 
ce 
the olfactory ganglions of many 
good examples of the nervous masses to whi 
allusion is here made. ny 
Tuber cinereum : its relation to the 
optic nerves. og 
The same difficulties uniformly encountered 
ae 
in all attempts to determine the particule 
functions of individual parts of the brain pr 
vail in the case of the tuber cinereum : s¢ 
physiologists maintained that the optic nerve 
derive a great number of filaments from tha 
body, and that the nerves are considerably ir 
creased in dimensions by this addition: Gal 
for instance, was. of this opinion, and gave 
rather exaggerated representation of the enla 
ment supposed to arise out of this reinforcer 
to the nerves. eG 
The optic nerves in man may doubtle 
draw some of their roots from the tuber cit 
reum, but there is an absolute certainty that 
body in question has other and probably 
important functions than any connected 
the origin of the nerves of vision. P 
and ven preeriare observations upon the sub 
are still a desideratum, but in the absences 
more direct evidence the anatomy of the mi 
brain is calculated to throw some light u 
the enquiry. In the mole the optic ne 
either wholly absent, or if t, it is m 
rudimental; nevertheless, the tuber cinere 
of enormous dimensions; it extends forw 
the olfactory lobes, and so far backwe 
nearly to reach the pons. In this animal, 
fore, there is an inverse proportion app 
cad 
4 
a fact little favourable to the hypothesis 
cated by Gall. ; 
Of the chiasma of the optic 
word chiasma (from the Greek ysacpeos, t 
satio,) means in strictness a decussatic 
crossing at acute angles, like the legs” 
letter X;* and, for convenience-sake, th 
expression (with a like latitude of app 
will be here employed to designate th 
responding structure in the lower animal: 
The organization of the human chias 
abundantly exercised the ingenuity ¢ 
mists, who seem to have encountere 
difficulty in their attempts to trace th 
filaments through it; and consequer 
withstanding all the attention w 
the subject, opinions the most conflie 
prevailed upon the true nature of the: 
in question. In no other instance is; 
junction between two corresponding 
opposite sides known to fe) 
anomaly affords strong presumptive e¥ 
the existence of some unusual properti 
... 
* In Human Anatomy the term is used | 
perhaps sufficient regard to its e log 
press the nervous mass in which t 
nerves are conjoined, ’ 
—— 
