CRANIAL NERVES. 



826 



Pathology, The motor oculi is frequently a sufferer by reason 

 of its situation and course. It is often compressed by tumors at 

 the base of the brain. In its passage through the sinus cavernosus 

 it is exposed to compression by a thrombosis of this venous canal. 



The course of the third nerve through the interpeduncular space 

 makes it play a considerable part in pathology. This is the place of 

 predilection for meningitic deposits. This segment of the nerve is 

 most frequently compressed in the exudates of tubercular meningitis. 

 It is also the point of attack of constitutional syphilis, particularly 

 during the tertiary period; this is a chronic meningitis which has 

 its principal focus at the interpeduncular space as an exudate. Diph- 

 theritic infection often attacks the third pair of cranial nerves. 



Paralysis of the oculomotor gives rise to external squint. Its 

 irritation causes internal squint, and also contraction of the pupil, 

 or myosis. The eye deviates outward in paralysis, due to the action of 

 the external rectus not being antagonized by the internal rectus. 



Diplopia. The deviation of one of the eyes does not permit the 

 maintenance of parallelism of the visual axes. Without this coinci- 

 dence the two images will not fall upon identical points in the retina. 

 Hence all objects seen will be double. This symptom, known as 

 diplopia, renders the sight very uncertain and often produces vertigo. 



Should the paralysis be general, so that it comprises the elevator 

 of the lid, Nature brings for itself a remedy for the defect of diplopia 

 by suppressing the vision of one eye. It does this by letting the lid 

 fall over the deviating eye. This drooping of the lid gives the con- 

 dition known as ptosis. 



Stimulation of the motor fibers of the third can be produced 

 reflexly by teething or intestinal irritations of children; hence their 

 squint. Chronic spasms of the eye-muscles which are involuntary 

 are called by the name nystagmus. 



Drugs. Atropine paralyzes the intra-ocular ends of the motor 

 oculi; Calabar lean stimulates them or paralyzes the sympathetic. 



FOURTH PAIR, OR PATHETIC NERVE. 



Distribution. The pathetic supplies the superior oblique muscle. 



Physiology. If the peripheral end of the pathetic be electrically 

 irritated, the superior oblique muscle contracts and turns the eyeball 

 downward and outward. 



The pathetic is a nerve that is especially endowed for the realiza- 

 tion of simple vision with the two eyes in inclined positions of the 

 head. It is impossible for an individual to carry one eye downward 



