310 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



From the olfactory bulb, after a relay, the olfactory impulses pass along the 

 olfactory tracts to certain parts of the brain described above. The olfactory 

 nerve is a pure sensory nerve. 



b) The second or optic nerve: The optic nerve arises in the retina of the eye 

 and passes through the coats of the eye, emerging ventral to the internal rectus 

 muscle. Find it there on the intact eye by pressing this muscle against the eye- 

 ball. It is a stout, white trunk which pierces the cartilage of the orbit and passes 

 to the ventral side of the diencephalon. It may be seen here by gently raising 

 the diencephalon. The optic nerve is not really a nerve, for the retina in which 

 it arises is a part of the brain wall, and in the retina there are several relays of 

 nerve cells between the rods and cones and the cells of origin of the optic nerve. 

 The optic nerve is really a tract of the brain. It carries sensory impulses only, 

 visual impulses, and discharges them into certain parts of the diencephalon and 

 optic lobes. 



c) The fourth or trochlear nerve: The trochlear nerve arises in the midbrain 

 and emerges in the groove between the optic lobes and the cerebellum. Trace 

 it on the side where the eye is still intact. It passes forward in the cranial cavity 

 to about the level of the cerebral hemispheres ; it then turns abruptly laterally, 

 pierces the wall of the orbit, and is distributed to the superior oblique muscle 

 of the eyeball. It is the motor nerve of this muscle and carries only somatic 

 motor impulses. Although it appears externally to emerge from the roof of the 

 midbrain, the motor cells from which it originates are in fact in the floor of the 

 midbrain in a forward extension of the somatic motor column. 



d) The third or oculomotor nerve: The oculomotor nerve arises from the floor 

 of the midbrain. It is readily noticed in the skate, ascending to the orbit near 

 the preceding nerve. In the dogfishes it is deeply situated and is seen by pressing 

 the cerebellum away from the wall of the orbit. Follow it into the orbit on both 

 sides, getting its general relations first on the side where the eyeball was removed. 

 It emerges into the orbit very near the insertion of the superior rectus muscle 

 and is situated ventral to the superficial ophthalmic nerve, already noted. It 

 should not be confused with the deep ophthalmic nerve which is in contact with 

 it as it enters the orbit; the deep ophthalmic nerve runs through the orbit in 

 contact with the medial surface of the eyeball. This nerve will be better seen 

 on the intact side. Observe the branches given by the oculomotor nerve immedi- 

 ately after its entrance into the orbit to the internal and superior recti muscles. 

 On the intact side now loosen the eyeball and cut the insertions of the superior 

 oblique and superior rectus close to the eyeball. Identify the deep ophthalmic 

 nerve passing in the spiny dogfish dorsal to the internal rectus lying against the 

 eyeball and in the smooth dogfish and skate ventral to the internal rectus. Free 

 and preserve this nerve. Cut through the insertion of the inferior rectus and 

 the optic nerve and, pressing the eyeball outward, note the branch of the oculo- 

 motor nerve, which passes along the posterior side of the inferior rectus muscle, 



