160 PAUL S. McKIBBEN 



In two cases, the trochlear nerve running cephalad in the dura 

 mater, left the cranial cavity through the olfactory foramen, 

 turning then sharply laterad to reach the superior oblique muscle. 



During its course inside the cranial cavity the trunk of the 

 nerve frequently splits into several fascicles. This splitting usu- 

 ally occurs in the region of the loop of the nerve or soon after 

 its origin from the brain and before the lateral cranial wall is 

 reached. 



After reaching its foramen the trochlear nerve runs laterad and 

 cephalad in its canal in the parietal bone for a distance of about 

 2 mm. (fig. 3). Reaching thus the dorsal face of the lateral 

 process of the parietal bone, the nerve runs cephalad close to 

 the bone from which the temporal muscle is arising. Having 

 arrived at the cephalo-medial angle of the orbit, the nerve swings 

 laterad and breaks up on the dorsal face of the superior oblique 

 muscle. 



In one animal the nerve on the right side joined the oculo- 

 motor nerve, leaving the cranium through the oculomotor fora- 

 men. The fibers of the two nerves intermingled so that it was 

 impossible to separate them. In this case the superior oblique 

 muscle was innervated by a branch of the dorsal ramus of the 

 oculomotor nerve, presumably by trochlear fibers. 



An anastomosis of the trochlear nerve has been observed in 

 several cases with the medial branch of the r. ophthalmicus pro- 

 fundus V inside the orbit (fig. 5); and in several cases a twig 

 from the same branch of the trigeminal nerve has been seen to 

 enter the sheath of the superior oblique muscle near to its 

 insertion on the eyeball. 



N. abducens 



The abducent nerve is formed by two or three twigs which 

 rise from the ventral face of the medulla oblongata about 0.3 

 mm. from the midline and 0.5 to 0.8 mm. cephalad to the root of 

 the glossopharyngeal nerve (fig. 2). The abducent nerve is the 

 smallest of the cranial nerves and contains usually ten to fifteen 

 fibers. Running at first directly laterad, the nerve swings cepha- 

 lad to the ventral surface of the root of the facial and acustic 



