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EXPLANATION OF PLATE XVL 
Fic. 1. Commissure of the optic nerves in the human body, examined after the 
nervous fibres had been hardened by alcohol. From Mayo’s Engravings. A A, Op- 
tic nerves. B B, Tractus optici, or continuation of the nerves behind the commissure. 
C, Semi-decussation. 
Fic. 2. Course of the Tractus optici, examined after similar preparation, in the 
sheep. A, Part of the right hemisphere of the brain. B, Olfactory lobes. C, Right 
Tractus opticus just behind the commissure. D, Crus cerebri. E, Tuber annulare. 
F, Right Tractus opticus cut across and torn backwards, as it expands upon and winds 
round the Crus cerebri and back of the Thalamus nervi optici, H, to be implanted on G, 
The upper portion of the right division of the Corpora quadrigemina, or optic lobes. 
Part of the fibres at this, their posterior termination, are turned back, to shew that 
those which lie lowest in the Tractus opticus are implanted in the highest part of 
the optic Lobe, and those which lie on the inner side of the former, are implanted in 
the outer side of the latter. I, Medulla oblongata. K, Cerebellum. L, Implanta- 
tion of the fifth nerve (7. e. the nerve of touch of the face and eye) on the side of 
the tuber annulare, in which no such contortion is observed. 
Fic. 3. Arrangement of the fibres of the optic nerves as they are implanted in 
the optic lobes of the fish, from Srrres’ Anat. Comp. du Cerveau, &c. <A, Cerebral 
lobes. B, Optic lobes laid open from behind. C, Cerebellum. 
Fic. 4. Decussation of the fibres which descend from the brain, through the 
Crura cerebri and Corpora pyramidalia, to the spinal cord in the human body ; 
found also, in a greater or less degree, in all the mammalia and birds, therefore co- 
existent in the Animal Kingdom, with the partial decussation of the optic nerves 
(Fig. 1). A, Corpus olivare. B, Corpus pyramidale. C, Decussating fibres. D, 
Spinal cord. 
