The Orbit. 443 



dentition, etc., in children may cause by reflex action a 

 form of strabismus strabismus spasticus due to a spasm 

 of the muscles, the result of stimulation of this nerve. 



The fourth nerve arises from the floor of the Sylvian 

 aqueduct near the centre for the third nerve and supplies 

 the superior oblique muscle. This muscle draws the eye- 

 ball downwards, and at the same time, rotates it inwards, 

 and hence, when paralyzed, the patient will be unable to 

 move the eyeball in a direction downwards and outwards, 

 so that, when going downstairs, for instance, the axes of 

 the two eyes will not be in the same plane, and diplopia or 

 double vision will result. 



Fifth merve. When the ophthalmic or sensory divi- 

 sion of the fifth nerve is injured, there is loss of sensation 

 in the eyeball itself, since the nasal, a branch of this nerve, 

 supplies the lenticular ganglion with sensory fibres, and 

 branches from this ganglion are distributed to the 

 eyeball. In injury to the ophthalmic, the conjunctiva, 

 with the exception of that portion which covers the lower 

 lid and is supplied by the infraorbital, is insensitive, and, 

 hence, foreign bodies may lodge and remain on the con- 

 junctiva of the eye, without the patient being aware of 

 their presence, until they mechanically cause inflammation 

 and probably ulceration ( since the trophic nerves are also 

 affected) of the conjunctiva and cornea. In sensory 

 paralysis, secretion of tears is interfered with, since the 

 lachrymal nerve, a branch of the ophthalmic, would be in- 

 volved. Sensation is wanting over the. forehead ( supraor- 

 bital and supratrochlear) ; in the region of the lachrymal 

 sac and side of the nose (infratrochlear) ; in the mucous 

 membrane of the front part of the nasal fossae and in the 

 skin over the lower part of the nose itself (nasal branch). 

 Normally, the presence of particles of dirt, etc, on the con- 

 junctiva induce, by reflex action, increased flow of tears 



