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with the right orbito-nasal ; whereas it actually runs on what is morphologically the 

 ventral side of the left eye, crossing in its course the left olfactory and orbito-nasal 

 nerves. But the explanation of this apparent anomaly is not difficult. The dorsal 

 branch of the fifth is a sensory nerve and was connected in the original symmetrical 

 fish with the skin of the extremity of the dorsal fin which was originally posterior to 

 the eyes. The fin remained behind the eyes during the rotation of the latter, and 

 after the left eye had travelled round to the right side, the dorsal fin with the neigh- 

 bouring muscles began to extend forwards. But instead of extending forwards along 

 the now distorted median dorsal line, the fin grew forward along the edge of the left 

 ectethmoid bone, which supports the left eye in its new position, and which is mor- 

 phologically ventral to the left eye. The nerve connected with the fin necessarily 

 accompanied the latter in its growth, and thus this nerve comes to be actually dorsal 

 and morphologically ventral to the left eye. The origin of the nerve remains in its 

 original position posterior to the eyes on the right side of the skull. 



The corresponding nerve of the left side makes its exit from the skull from a corres- 

 ponding foramen in the flat proximal portion of the left frontal bone, on the lower (left) 

 side of the dorsal fin ; it passes forwards on the left side of the fin to its apex. 



On the right side the trunk of the fifth nerve, which emerges from the skull through 

 the sphenotic foramen, divides immediately after its exit into two large branches, the 

 maxillary and mandibular nerves. The maxillary branch supplies with sensory fibres 

 the skin of the upper jaw and also sends motor fibres to the palatal muscle. The 

 mandibular branch supplies with sensory fibres the skin of the lower jaw; the largest 

 division of it enters a canal in the mandibular bone, whence it supplies the mucous 

 membrane of the floor of the mouth. The main trunk of these two branches, the 

 maxillary and mandibular, while still within the skull, gives off a small nerve which 

 leaves the skull by the great anterior opening, and passes forwards below the recti 

 muscles of the eye ; it supplies the anterior part of the palate and is called the palato- 

 nasal. 



On the left side these branches of the fifth are much enlarged, their sensory fibres 

 being multiplied in proportion to the great development of tactile filaments and 

 epidermic sense-organs on this side. The palato-nasal branch reaches this side of the 

 head partly through the gap between the ectethmoid and parasphenoid, partly through 

 a foramen in the sphenotic in front of the large sphenotic foramen. The latter foramen 

 is much larger than that on the right side, and the common trunk of the maxillary and 

 mandibular nerves which passes through it is proportionally large. These branches of 

 the fifth on the left side are represented in PI. XV, 2, where the maxillary is indicated 

 by Y 2, the palato-nasal by V 2a, and the mandibular by V 3. The dorsal branch 

 of the fifth, which accompanies the dorsal fin, is seen passing forwards just above the 

 palato-nasal. 



The seventh cranial nerve is called the facial ; its distribution is the same on both 

 sides, as it belongs to a region of which the symmetry has not been disturbed. The 



