514 THE NERVOUS SYSTEM. 



into play. If the recti act alternately, the eyeball will be rotated 

 completely, as in looking around a room from one side to the 

 other and back again, from the floor to the ceiling. The motor 

 oculi is purely a motor nerve. When stimulated, contraction is 

 produced in the muscles to which it is distributed ; when the 

 nerve is divided, these muscles are paralyzed. 



Paralysis of the Motor Oculi. When the motor oculi is par- 

 alyzed, the following are the results : 



(a) External strabismus, which consists in a turning of the eye 

 outward. The retention of the eye in its normal position requires 

 the conjoint action of the internal and external recti. In paralysis 

 of the motor oculi the internal rectus has lost its innervation, 

 and therefore its power to contract, and the external rectus, 

 which receives its nervous supply from another nerve (the ab- 

 ducens), having lost its antagonist, turns the eye outward. 



(6) Luscitas. After external strabismus has been produced the 

 eye remains in that condition, for the muscles which could move 

 it in any other direction have been paralyzed. This immobility 

 is called " luscitas." 



(c) Ptosis. The levator palpebrse superioris is also paralyzed, 

 and the upper eyelid droops, constituting ptosis. The ability to 

 close the eye still remains, as this is the act of the orbicularis 

 palpebrarum, which is not innervated by the third, but by the 

 seventh, nerve. 



(d) Mydriasis. A branch of the motor oculi goes to the ciliary 

 ganglion, which gives off the ciliary nerves that supply the iris. 

 Accompanying the manifestations of paralysis of the motor oculi 

 already mentioned there is in addition a dilatation of the pupil, or 

 mydriasis. The diminution of the size of the pupil following the 

 action of light upon the retina does not take place when this nerve 

 is paralyzed. The contraction of the pupil is a reflex act requiring 

 the integrity of the optic nerve, which serves as a carrier of the 

 luminous impressions to the brain, and of the motor oculi, which 

 is the efferent nerve in this act. 



(e) Inability to Focus. The muscle concerned in focusing the 

 eye for short distances is the ciliary. The power to focus is lost 

 in paralysis of the motor oculi. Paralysis of the motor oculi may 

 be due to disease of the brain or to pressure on the nerve. If 

 the trunk of the nerve is affected, all the physical signs mentioned 

 may be observed, while if a single branch only is involved, the 

 effect will be limited to the part to which that branch is dis- 

 tributed. 



Trochlearis. The apparent origin of the trochlearis or patheti- 

 cus is on the outer side of the crus cerebri, in front of the pons, 

 and its real origin is a nucleus continuous with that of the motor 

 oculi. The trochlearis leaves the cranium by the sphenoidal 

 fissure, and is distributed to but one muscle, the superior oblique. 



