54 THE NERVOUS SYSTEM 



side of the body. The condition of quadrantic hemianopia 

 in lesions of the occipital cortex has already been described 



(P- 13)- 



The effects which result from pressure on the optic chiasma 

 are by no means constant. Homonymous or heteronymous 

 hemianopia or complete blindness in one or both eyes may 

 occur. Bitemporal hemianopia occurs in many cases of 

 acromegaly, and it is due to pressure on the optic chiasma by 

 tumours of the hypophysis (pituitary body), which is related 

 to its posterior aspect (p. 401). 



The optic chiasma, the posterior ends of the optic nerves, 

 and the anterior ends of the optic tracts are all situated in the 

 cisterna interpeduncularis (p. in). On this account they are 

 often involved in basal meningitis, and the ensuing symptoms 

 may be very irregular. 



The "Argyll-Robertson pupil," which is an important early 

 symptom of locomotor ataxia, consists in the loss of the light 

 reflex, although the pupillary reactions to convergence and 

 accommodation remain normal. Since vision is not affected, 

 it is not improbable that the condition may be caused by a 

 lesion affecting the pupillary fibres, after they leave the optic 

 tract and before they reach the iris nucleus (Fig. 28). 



The THIRD or OCULOMOTOR, the FOURTH or TROCHLEAR, 

 and the SIXTH or ABDUCENT NERVES are all purely motor 

 in function, and their distribution is restricted to the muscles 

 of the orbit and of the eyeball. 



NOTE. It is convenient to describe the course and relations of these 

 nerves and to indicate the actions of the orbital muscles, before the distri- 

 bution of the nerves and the results of pathological lesions are dealt with. 



The Oculomotor Nerve arises from a nucleus which is 

 placed in the upper part of the mid-brain in the grey matter 

 which surrounds the cerebral aqueduct (Fig. 10). From 

 the nucleus the fibres pass forwards through the mid-brain 

 and emerge on its anterior surface. The nerve traverses the 

 cisterna basalis and pierces the dura mater to the lateral side of 



