Double and Inverted Images on the Retine. 481 
of Newron is still defective. Several additions to our knowledge 
on the subject have been made since the time of Reip. 1. The 
supposition of the semi-decussation of the optic nerves, and con- 
sequent connexion of half of the retina of each eye with one of 
the optic lobes in the human body, has acquired additional pro- 
bability from the observation, first applied to the subject by Dr 
Wo vasrton, of that form of temporary loss of vision formerly 
called Suffusio dimidians, in which, although both eyes are open, 
one-half of the field of vision disappears from the sight ; which 
is easily conceivable on the supposition of the right half of each 
retina being connected with the same portion of the sensorium, 
and not easily understood on any other supposition.* 2. That 
the commissure of the optic nerves is really a partial decussation 
in all animals where it exists, seems to me, if not absolutely ascer- 
tained, to be put on very strong ground, by the dissections of 
Mayo and the observations of SERRES. 
Of the dissections of Mr Mayo, made after the nervous fibrils 
were hardened by alcohol, in the human body, we have a distinct 
record in his engraving of the commissure of the optic nerves 
(Pl. vii. fig. 3. of his Engravings ; Fig. 1. of Plate X VI. hereto an- 
nexed), of the accuracy of which I am satisfied, from some trials 
which I have myself made in the same manner, and in which it dis- 
tinctly appears, that the outer portion of each optic nerve is 
formed by fibres proceeding forward from the outer part of 
the tractus opticus of the same side, and that some fibres, at 
least, of the inner portion of each optic nerve, have crossed over 
from the inner part of the tractus of the opposite side, as sup- 
posed in this theory. 
The result of the inquiries of Serres on this: point, in various 
animals, is stated as follows. He never could satisfy himself on 
the subject by preparations either of the human body, or others 
* This affection is sometimes quite transient ; but in other cases, as I have my- 
self known, it is permanent, at least for weeks together. 
