COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 347 



of origin of the external rectus from the orbit. The larger is the oculomotor, the 

 smaller the abducens. Cut through the insertions of the inferior oblique and 

 inferior and external recti at the eyeball and deflect them ventrally. Above the 

 inferior rectus the internal or medial rectus will be seen inserted on the eyeball. 

 Look on the inner surface of the external rectus and find the abducens nerve, 

 curving around the posterior border of the origin of this muscle and passing on- 

 to its surface. Return to the dorsal surface of the eyeball, cut through the inser- 

 tion of the superior oblique at the eyeball, and press the eyeball ventrally. Two 

 nerves will be seen on the medial wall of the orbit. The lower one is the trochlear 

 nerve. Trace it to the medial surface of the superior oblique. The upper nerve 

 is the frontal nerve, one of the main branches of the ophthalmic branch of the 

 trigeminus. It passes to the dorsal part of the orbit and exits through the 

 anterior supraorbital foramen to be distributed to the upper eyelid and skin in 

 front of the orbit. It may have been cut in removing the supraorbital arch. 

 The white part of the Harderian gland will be noted in the anterior part of the 

 orbit. Cut through all of the insertions of the eye muscles at the eyeball and 

 through the optic nerve, removing the eyeball. The optic nerve is the stout 

 white trunk near the superior rectus. The muscles around the optic nerve, 

 exclusive of those already identified, belong to the retractor bulbi. Find the 

 main trunk of the oculomotor nerve and trace its branches to the retractor 

 bulbi and superior and internal recti. The main nerve curves below the optic 

 nerve. The nasociliary branch of the ophthalmic nerve may be noted passing 

 between the superior oblique and the retractor bulbi. Its main portion, the 

 ethmoidal nerve, leaves the orbit by a small foramen in front of the superior 

 oblique muscle. On tracing this nerve posteriorly fine branches to the orbit 

 may be seen. 



Trace the nerves of the orbit to their exits from the skull. The third, fourth, 

 and sixth nerves and the ophthalmic and maxillary branches of the trigeminus 

 pass through the orbital fissure. The mandibular branch of the trigeminus 

 passes through the foramen lacerum. 



Cat: Remove the eyelids and the surrounding skin, cutting them away from 

 the eyeball. Remove the half of the mandible and the zygomatic arch from 

 the side on which you are working. Press the eyeball ventrally away from the 

 supraorbital arch. In the anterodorsal angle of the orbit a strong fibrous con- 

 nection will be found between the wall of the orbit and the eyeball. On investi- 

 gating this it is found to consist of two fibrous bands which form a pulley; this 

 is known as the trochlea. The tendon of the superior oblique muscle passes over 

 the trochlea and is inserted on the eyeball. Its insertion is much expanded and 

 extends caudad from the trochlea. Posterior to the insertion of the superior 

 oblique is a thin flat muscle, the levator palpebrae superioris, or elevator of the 

 upper eyelid. This passes to the dorsoposterior surface of the eyeball. Cut 

 through this at its insertion. Posterior to this muscle in the dorsoposterior 



