SUMMARY. 235 



lution of the intercedent region. It gives origin on its outer side to the smaller portion 

 of the fifth and the hard portion of the seventh, and on its inner side to the sixth, and 

 by its extension downwards to the ninth. Part of it resembles the olivary body, and in 

 all other respects, except in form, it is analogous with the most posterior or oblique 

 tract of the annular tubercle in man, which is the continuation of the tract of the larger 

 portion of the intermediate layer of the exterior region of the brain after it has received 

 the tract of the convolution of the intercedent region. This oblique tract of the annular 

 tubercle on its outer side also gives origin to the smaller portion of the fifth, and lower 

 clown to the ninth, and in the lower part of its median side it gives origin to the sixth 

 nerve, and its continuation down the oblong medulla includes the olivary body, and 

 from the inner side of this gives origin to the ninth. The trapezoid body is not 

 distinct in birds, and does not exist in amphibia and fishes. The oblong medulla is 

 larger in mammalia in proportion to the size of the brain than in man, but par- 

 ticularly in the other classes ; small olivary bodies exist in the monkey ; the other 

 eminences in the four superior classes are more or less indistinct. 



The olfactory nerve, except in animals not having a nose, exists in the four 

 superior classes ; it arises from the anterior lobe of the brain when it is small, but 

 extends further backwards when it is large ; it is of small size in man and simise, and 

 very large in mammalia generally ; it varies in the form or absence of the ganglion in 

 birds and amphibia, and the shape of the ganglion in fishes. The optic nerve exists in 

 the four superior classes and many of the invertebrate ; in the three superior the tract 

 is connected with the thalamus, and in the four with the optic lobe, and in mammalia 

 and birds with the true visual tract, and with the mammillary eminence, or the promi- 

 nence in the place of it, and in mammalia with the geniculate body. The commissure 

 of the two tracts varies ; it is large in man, less in mammalia, and still less in amphibia 

 and fishes ; there is an interchange of fibres from each tract in it, but in some fishes 

 there is a complete decussation of the two nerves. The third nerve arises in man 

 and mammalia from the inner side of the crus of the brain, and further from the 

 smaller portion of the intermediate layer of the exterior region of the brain ; in birds, 

 amphibia, and fishes, from the oblong medulla, near the same place ; in all, it supplies 



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