IX BRAIN, CRANIAL NERVES 333 



Dogfish it can hardly be said to exist as a discrete nerve, owing to the 

 olfactory organ and olfactory bulb being practically in contact, the 

 diffuse nerve-fibres passing across directly from one to the other. 



Optic (Fig. 138, II). — The Second Cranial nerve is also purely sensory. 

 It is the nerve of sight and conveys impulses from the eye in towards 

 the brain. It is a very thick nerve and passes into the floor of the 

 thalamencephalon. In some fishes the right and left nerves cross like 

 an X, the nerve from the right eye passing towards the left side of the 

 brain and conversely. In the Dogfish and in the great majority of 

 vertebrates the crossing (optic chiasma) is of a much more complicated 

 character, some fibres passing from the eye to the opposite side of the 

 brain, some to the same side of the brain : others pass from one eye to 

 the other eye, and still other fibres pass outwards in the one root from 

 the brain and then double back into the brain by the other root. 



Oculomotor, Abducent, Pathetic. — The Third, Fourth and Sixth 

 cranial nerves may conveniently be taken together. They are all motor 

 nerves which supply the muscles of the eyeball. The eyeball of the 

 vertebrate is contained within a socket — ^the orbit — in which it can be 

 freely moved so as to direct it in one direction or another. These move- 

 ments are carried out by muscles one end of which arises from the wall 

 of the orbit while the other is attached to the surface of the eyeball. 

 Throughout the whole series of vertebrates with functional eyes the 

 muscles attached to each eyeball are precisely the same and are six in 

 number. Four of these radiate out from a point on the inner wall of 

 the orbit to opposite sides of the eyeball. These are the Recti and they 

 are individually named, from their position in the human body. External 

 (Fig. 139, e.r — in the Dogfish posterior in position as the eye looks 

 outwards instead of forwards). Internal {i.r), Superior (s.r) and Inferior 

 Rectus (Fig. 139, B, inf.r). The other two muscles pass from the anterior 

 wall of the orbit to the upper and lower side of the eyeball respectively 

 and are known as the Superior Oblique {s.o) and the Inferior Oblique 

 (Fig. 139, B, i.o). 



Pathetic (Fig. 138, IV). — The Fourth cranial nerve serves simply as 

 the motor nerve for the Superior Oblique muscle. It is accordingly of 

 relatively small size and is peculiar among the cranial nerves in that it 

 arises from the roof of the brain — in the angle between cerebellum and 

 mesencephalon. 



Abducent (Fig. 138, B, VI). — ^The Sixth nerve similarly is the 

 motor nerve of a single eye-muscle — the Exteriial Rectus. It also is 

 of very small size : it arises from the ventral surface of the medulla 

 oblongata. 



