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EXPLANATION OF PLATE XVI 



Fig. 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. 



Fig. 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 he 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 (i. e. the nerve of touch of the face and eye) on the side of 

 the tuber annulare, in which no such contortion is observed. 



Fig. 3. Arrangement of the fibres of the optic nerves as they are implanted in 

 the optic lobes of the fish, from Serres 1 Anat. Comp. du Cerveau, &c. A, Cerebral 

 lobes. B, Optic lobes laid open from behind. C, Cerebellum. 



Fig. 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. 



