580 SIGHT. 



declared the opposite opinion on account of the existence of the 

 posterior quadrigemina in animals apparently destitute of olfac- 

 tory nerves. He believed that the posterior reinforced and pro- 

 tected the optic nerve as well as the anterior, but in a different 

 manner, since in different animals they are in different propor- 

 tions, and the posterior scarcely perceptible, if not absent, in 

 some whose vision is perfect. Nay, he believed that the greater 

 part of the corpora quadrigemina have other offices relating to 

 the spinal chord below and to other cerebral parts than those con- 

 cerned in vision above. If the surface either of their anterior part, 

 of the bulb of the olfactory nerve, of the grey band on the sides 

 of the fourth ventricle, or of the ganglion of the acoustic nerve, is 

 touched, no contraction ensues ; but injure them to a certain depth, 

 and convulsions take place. 8 The optic nerve is certainly not 

 derived, he says, from the tubercles only ; the internal and ex- 

 ternal corpora geniculata, &c. give it many filaments. 



Many recorded cases, as well as many in my own practice, 

 prove that one half of the retina may be paralysed, while the 

 other half remains unaffected ; and this effect may be common 

 to both eyes, or peculiar to one. Dr. Wollaston l relates that 

 it twice occurred to him not to be able to see but on one side 

 of the axis of vision. The first time, the left side of each eye 

 was affected ; he saw but the half of a man's face or of any 

 object he looked at ; and, in attempting to read the name JOHN- 

 SON over a door, he saw only .... SON, the commencement 

 of the name being totally obliterated from his view : the com- 

 plaint was of short duration. About nineteen years afterwards 

 the phenomenon recurred: this time, the right side of the eye, 

 about three degrees from the centre of the retina, was affected, 

 and its duration was ten minutes. Two analogous cases are also 

 mentioned by Dr. Wollaston. Desmoulins u states that M. Arago 

 has experienced this affection of vision three times : the first two 

 times, objects situated to the right of the axis of vision were in- 

 visible ; the third time he saw objects on the right only of this 

 axis. The same author notices also the following remarkable 

 case. In consequence of a cerebral fever, the external side of the 

 left retina of M. de M became insensible : with this eye he 



I.e. 8vo. vi. p. 253. * Phil. Trans. 1824. 



u Anatomic des systemes nerveux des animaux a vertebres, t. ii. p. 673, 



