no 



the brain behind tlie origins of the optic nerves, and pass from the slvull 1)y apertures 

 in the membrane wliich closes its anterior opening. 



The fifth nerve or trigeminal arises by several roots from the side of the medulla 

 oblongata below the cerebellum : in its origin it is closely connected with the seventh 

 or facial nerve. In fact the two nerves arise from a number of roots common to both, 

 of which the dorsal are sensory, the ventral motor ; the fifth nerve is chiefly composed 

 of fibres from the sensory roots, with the addition of some from the motor, while the 

 facial consists chiefly of fibres from the motor roots, with the addition of some iiom 

 the sensorj'. 



The fifth nerve consists of a large number of branches. It does not leave the skull 

 by a single trunk, but divides on the inner wall of the skull into several, the largest 

 and most important of which leaves the skull after a very short course by the large 

 trigeminal foramen in the lower part of the sphenotic bone. The other two branches 

 run for some distance on the inner wall of the skull before emerging ; one of them runs 

 directly forwards and emerges through the anterior membrane wliich separates the 

 skull from the orbits ; this forms the orbito-nasal nerve ; the other curves upwards arid 

 forwards and emerges by a small foramen in the frontal bone. As the corresponding 

 nerves difier on the two sides it will be necessary to describe them separately. 



The course of both orbito-nasal nerves, like that of the two olfactory, is to be followed 

 by dissection of the upper side of the head ; for each orbito-nasal nerve lies in close 

 relation to the olfactory, lying in the ordinary fish dorsal to the olfactory and between 

 the eye muscles and the interorbital septum. Accordingly in the sole the orbito-nasal 

 nerves are found on the upper side of the head, one on either side of the interorbital 

 septum. But the dorsal (left) is much larger, and longer than the ventral (right). The 

 left nerve passes over the surface of the mesethmoid bone at the bottom of the rounded 

 notch between the left ectethmoid and the mesethmoid, and then enters the gelatinous 

 tissue of the end of the snout, sends branches to the skin of the upper side at this part 

 of the snout, but is chiefly distributed to the tactile filaments of the skin on the lower 

 side between the olfactory capsule and the edge of the snout. The right nerve just 

 behind the right ectethmoid passes into a canal between the right frontal bone and the 

 right ectethmoid, and thence emerges again on the right upper surface of the meseth- 

 moid, enters the gelatinous tissue of the snout, and supplies the small area of skin on 

 the upper side between the mouth and the apex of the snout. 



The right dorsal branch of the fifth, after its upward course on the internal surface 

 of the skull, emerges by a small foramen in the flat proximal portion of the right frontal 

 bone, and thence passes forwards, at some depth from the surface, between the cephalic 

 portion of the lateral muscle and the membrane which forms the dorsal boundary 

 of the dorsal (left) orbit. It supplies the skin of the extreme anterior end of the 

 dorsal fin on the upper side. It seems at first sight that this nerve has changed its 

 morphological relations ; for since it belongs to the right side of the head we might 

 expect to find its anterior part on the ventral or right side of the interorbital septum, 



