188 CLINICAL APPLIED ANATOMY. 



foramen are the first to suffer, giving rise to concentric diminution 

 of the visual fields. 



The ophthalmic artery lies below and to the outer side of the 

 nerve in the intracranial portion of its course, but leaves the dural 

 sheath to curl over the nerve when the orbit is reached. 

 Aneurysms may compress the nerve in either situation. The 

 central artery of the retina which is sometimes the site of 

 embolism, passes into the nerve a short distance behind the 

 eyeball. 



The optic chiasma practically forms part of the floor of the 

 third ventricle, and so may be compressed by effusions in the 

 ventricular system. Immediately behind the chiasma lies the 

 pituitary body, tumours of which may give rise to a characteristic 

 form of visual defect, bitemporal hemianopia. The internal 

 carotid arteries occupy the outer angles of the chiasma and nasal 

 hemianopia may be produced by aneurysms or calcification of 

 these vessels, but is rare. The sphenoidal air sinuses are in the 

 floor of the middle fossa below the chiasma, and the latter lies in 

 the cisterna basalis so that it is liable to invasion by gummatous 

 and tuberculous tumours, and inflammation of the meninges. 



Disease processes which affect the chiasma act chiefly on its 

 central portion and accordingly the characteristic symptom of a 

 chiasma lesion is bitemporal hemianopia due to interference 

 with the decussating fibres from the nasal halves of the retina, 

 which in turn receive impressions from the temporal halves of 

 the visual fields. The temporal fibres are uncrossed, and chiasma 

 lesions often extend irregularly to one side or the other and so 

 may involve temporal fibres also in varying degree. (Fig. 16.) 



A lesion limited to the non-decussating (temporal) fibres of 

 one side should produce nasal hemianopia, but this is exceptional. 



The adjacent motor nerves of the eye are liable to be involved 

 in affections of the chiasma. 



The optic tract extends backwards from the chiasma to 

 terminate in the anterior corpus quadrigeminum, the external 

 geniculate body, and the pulvinar of the optic thalamus. In part 

 of its course it is closely applied to the fore part of the crus 



