12 MORPHOLOGY OF THE VERTEBRATA. 



branches to the upper lip ; it gives a large branch to the cellular 

 -structure at the side of the head. The third trunk passes behind 

 the muscles of the jaw, gives branches to these, and is distributed 

 on the skin about the mouth and gills ; one of its branches forms 

 -a ganglion, which gives off filaments to terminate about the 

 under lip. The auditory portion of the fifth passes to the ear, 

 and gives branches to the sacs containing the cretaceous matter 

 and the globular enlargements of the semicircular canals, whilst 

 a nerve similar to the glosso-pharyngeal communicates with it 

 on the large sac, and gives filaments to the termination of a 

 .semicircular canal, and then passes outwards to give filaments 

 on the surface of the first and the corresponding one of the 

 second division of the gills, and terminates on the membrane of 

 the mouth. The vagus is seen passing to the gills, giving fila- 

 ments to the muscular appendages of these, and then sending a 

 large branch to distribute filaments to every part of the opposite 

 surface of each division, except the first; it sends a branch 

 .anteriorly to the stomach, which communicates with the 

 sympathetic ; it sends a large branch posteriorly to pass under- 

 neath the muscles of the back, and give filaments to these in its 

 •course; this branch lies upon and crosses the spinal nerves; near 

 the tail it emerges from underneath the muscles, and passes just 

 under the skin to the extremity of this part. 



The optic nerves unite into a chiasma, but do not decussate. 



ORGANS OF SENSE. 



EAR. 



External aperture of, situated at margin of gape on under 

 surface of skull. 



The labyrinth— the essential portion of the organ of hearing-^ 

 is entirely enclosed within cartilage. The semicircular canals are 

 arranged in the ordinary manner, being three in number. 



NASAL SACS. 



Cavity of nasal chamber is double, each part opening by a 

 separate aperture on the under surface of skull at sides of mouth. 

 The interior of the olfactory chamber is lined by mucous mem- 

 brane, arranged in such a manner as to form numerous folds, by 

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