980 



THE NERVE SYSTEM 



this latter a short, thick branch, radix brevis ganglii ciliaris, is given off to the lower 

 part of the ciliary or lenticular ganglion and forms its short or motor root (Figs. 

 730 and 733). All these branches enter the muscles on their ocular surface, 

 except that to the Inferior oblique, which enters its posterior border. 



Applied Anatomy. Paralusis of the oculomotor nerve may be the result of many causes: 

 as cerebral disease; conditions causing pressure on the cavernous sinus; periostitis of the bone 

 entering into the formation of the sphenoidal fissure; fracture of the orbit. It results, when 

 complete, in (1) ptosis, or drooping of the upper eyelid, in consequence of the Levator palpebrae 

 being paralyzed ; (2) external strabismus, on account of the unopposed action of the External 

 rectus muscle, which is not supplied by the oculomotor nerve, and is not therefore paralyzed; 

 (3) dilatation of the pupil, because the sphincter fibres of -the iris are paralyzed; (4) loss of 

 power of accommodation, as the Sphincter pupillse, the Ciliary muscle, and the Internal rectus 

 are paralyzed; (5) slight prominence of the eyeball, owing to most of its muscles being relaxed. 



^Oculomotor 'Nerve 



From Cavernous Plexus 



FIG. 730. Plan of the oculomotor nerve. 



Occasionally paralysis may affect only a part of the nerve; that is to say, there may be, for ex- 

 ample, a dilated and fixed pupil, with ptosis, but no other signs. Irritation of the nerve causes 

 spasm of one or other of the muscles supplied by it; thus, there may be internal strabismus from 

 spasm of the Internal rectus; accommodation for near objects only from spasm of the Ciliary 

 muscle, or contraction of the pupil (myosis), from irritation of the sphincter of the pupil. 



The oculomotor nerve is particularly liable to become involved in a syphilitic periarteritis 

 where it passes between the superior cerebellar and posterior cerebral arteries; associated with 

 locomotor ataxia various partial or complete paralyses of the nerve are often seen. 



THE FOURTH OR TROCHLEAR NERVE (N. TROCHLEARIS) (Figs. 629, 633). 



The fourth or trochlear nerve is, with the exception of the n. intermedius, the 

 smallest of the cranial nerves, and supplies the Superior oblique muscle. 



It arises from a nucleus in the floor of the mid-brain aqueduct at the level of 

 the inferior quadrigeminal body. From its origin the nerve runs outward, curving 

 around the central aqueduct gray to turn inward and backward into the superior 

 medullary velum, decussating with the corresponding nerve of the opposite side 

 and emerging from the surface laterad of the frenulum veli, immediately behind 

 (or caudad of) the posterior quadrigeminal body. 



Emerging from the superior medullary velum, the nerve is directed outward 

 across the superior peduncle of the cerebellum, and then winds forward around 

 the outer side of the crus cerebri, immediately above the pons, pierces the dura 

 in the free border of the tentorium, just behind, and external to, the posterior 

 clinoid process, and passes forward in the outer wall of the cavernous sinus, 

 between the oculomotor nerve and the ophthalmic division of the trigeminal 

 nerve (Figs. 505 and 506). It crosses the oculomotor nerve and enters the 



