Motor Oatli 59 



same (the third) nerve. Also, when a near object is viewed, the rays 

 falling upon the eye are extremely divergent, and it is necessary that 

 the lens be rendered more convex to focus them ; this is accomplished 

 by the ciliary muscle, through the influence of the third nerve. Thus 

 the third nerve has entire charge of the accommodation for near vision ; 

 it converges the visual axes, contracts the pupil, and renders the front 

 of the lens more convex. 



Dilatation of the pupil is affected by radiating muscular fibres in 

 the iris under the control of the sympathetic. The influence of the 

 sympathetic upon the pupil, it may be remembered, is exactly the 

 reverse of that upon a blood-vessel when it is stimulated the pupil is 

 dilated, whereas a blood-vessel would be contracted. 



The 'Argyll Robertson pupil,' as a symptom of posterior spinal 

 sclerosis, is thus explained : when the eye of a healthy man is directed 

 upon a distant object the pupil is dilated, and when he looks at a 

 finger, in front of his nose, for instance, the pupil is contracted ; also 

 when the eye is in shadow the pupil is dilated, but under the influence 

 of a bright light it contracts. In the case of the light, contraction of 

 the pupil is reflex ; but in the former case it is effected in accommoda- 

 tion (vide supra}. Now, in sclerosis the pupillary reflex the contrac- 

 tion under the stimulus of light is abolished, whilst the accommoda- 

 tion-contraction remains. Many other reflexes in addition to those of 

 the pupil are lost in locomotor ataxy as the result of degenerative 

 changes in afferent fibres. 



When the third nerve is paralysed the upper eye-lid drops, the 

 levator palpebrae being unable to hold it up. The condition is called 

 ptosis (TTTCOO-I?, a falling ; 7rwrra>), and, the internal rectus being thrown 

 out of work, the external rectus holds the eye-ball in permanent 

 abduction (divergent squint}. The pupil is dilated and does not 

 contract to light, and accommodation for near vision is impossible. 

 Most of the muscles having relaxed their hold, the eyeball protrudes 

 between the Y\&sproptosis (77/30, forwards, Trroxnr, falling). It cannot 

 be tilted upwards, inwards, nor downwards. 



Likely causes of the paralysis are syphilitic inflammation or 

 deposit at the base of the brain, haemorrhage, tumour, and diphtheria. 

 When it is due to a lesion near the Sylvian aqueduct the paralysis 

 may be bilateral, and the fourth and the sixth nerves may be impli- 

 cated in due course. The roots of origin of the third nerve may also 

 be caught in a widely-spreading degeneration which causes labio-glosso- 

 pharyngeal paralysis (p. 56). 



Double vision occurs when the recti act out of harmony, because 

 the associated areas of the retinae cannot be simultaneously directed 

 upon the object. 



No. 4. The patheticus, a thread-like nerve, comes round the crus 

 from its origin at the valve of Vieussens, and, passing along in the 

 outer wall of the cavernous sinus, and through the sphenoidal fissure, 



