682 TROCHLEAR NERVE. 



sympathetic through the carotid plexus and indirectly through the abducens, 

 in this way receiving vasomotor fibers. The rare cases in which fibers for the 

 sphincter have been found in the abducens or even in the trigeminus must be 

 considered as examples of variations in the course of the pupillary fibers. 



The intraocular fibers of the oculomotor nerve are paralyzed by 

 atropin and stimulated by physostigmin (or the sympathetic is para- 

 lyzed, or both). 



Contraction of the pupils on irritation of the nerve can be best demonstrated 

 in the severed and opened head of a bird. Asphyxia, sudden cerebral anemia 

 (from ligature of the carotid arteries or beheading) , and likewise sudden venous 

 stasis, cause dilatation of the pupils, as in death, through paralysis of the oculo- 

 motor nerve. 



Pathological. Complete paralysis of the oculomotor nerve gives rise: (i) To 

 drooping of the upper lid (paralytic ptosis) . (2) To immobility of the eyeball. 

 (3) To rotation of the eye outward and downward (strabismus'], and as a result 

 to diplopia. (4) To slight protrusion of the bulb, because the superior oblique, 

 which draws the eye forward, is unopposed by the action of its antagonists, the 

 three paralyzed rectus muscles, which draw the eye backward. In animals, which 

 have a retractor muscle of the bulb, this symptom is more conspicuous. (5) To 

 moderate dilatation of the pupil (paralytic mydriasis) . (6) To inability on the 

 part of the pupil to contract upon stimulation by light. (7) To loss of the power 

 of accommodation of the eye for near vision. The paralysis naturally may be 

 confined to individual portions or be incomplete. Destruction of the posterior 

 portion of the oculomotor nucleus causes only paralysis of the external ocular 

 muscles (external ophthalmoplegia) . 



Irritation of the branch for the elevator of the lid causes spastic lagophthalmos 

 in man; of the other muscular branches, corresponding spastic strabismus. These 

 latter irritations may be induced also reflexly, as, for example, during dentition 

 and in association with the diarrheas of childhood. Clonic contractions manifest 

 themselves bilaterally as involuntary oscillation of the eyes (nystagmus) in conse- 

 quence of irritation of the quadrigeminate bodies. Tonic spasm of the sphincter 

 of the pupil is designated spastic myosis, clonic spasm hippus. Spasm of accom- 

 modation is also observed, and in conjunction with it not rarely macropia in con- 

 sequence of imperfect estimation of distance. 



IV. TROCHLEAR NERVE. 



The trochlear nerve arises by means of neurites from the ganglion-cells of 

 the trochlear nucleus, which is situated immediately behind the lateral chief 

 nucleus of the oculomotor nerve, and really forms a continuation of the anterior 

 horn (constituted of two sections joined together), below the gray matter 

 surrounding the aqueduct of Sylvius. It then passes to the lower border of the 

 posterior quadrigeminate body, and further on into the superior medullary velum, 

 and decussates with the root of the opposite side in the velum and then appears 

 free (Fig. 241). Like the third and sixth cerebral nerves it is probably connected 

 by fibers with the cortical motor center for the ocular muscles. 



The trochlear is the voluntary motor nerve of the superior ob'.ique 

 muscle. Its coordinated innervation, however, is involuntary. 



Its connections with the carotid plexus of the sympathetic and the first branch 

 of the trigeminus have the same significance as the analogous connections of the 

 oculomotor nerve. 



Pathological. Paralysis of the trochlear nerve causes only slight loss of the 

 mobility of the eyeball outward and downward, with the development of slight 

 rotation inward and upward and diploplia. The images are placed obliquely 

 one above the other, approach each other when the head is turned toward 

 the unaffected side, and are separated when the head is turned toward the affected 

 side. The patient at first inclines the head forward, but subsequently rotates it 

 about the vertical axis towaid the unaffected side. When the head is rotated, 

 the healthy eye retaining the primary position, the eye makes a similar movement. 

 Spasm of the trochlear nerve causes rotation of the eye outward and downward. 



