SECTION X 

 ERRORS OF APPRECIATION 



UNDER this heading we include all types of abnormality in the retinal 

 apparatus or in its central connections. The class therefore includes cases 

 in which the image formed on the retina is in every way normal, and those 

 in which the optical defects do not adequately explain the whole of the visual 

 disability, which experiment shows to be present. The class is found to 

 include cases which range from slight impairment to complete blindness. 



The following classification may be used: 



Group 1. Both rod and cone vision are affected, and there is thus 

 both night blindness and total colour blindness. 



Group 2. Rod vision is either affected alone, or there is slight defect in 

 cone vision as well. 



Group 3. Rod vision is unaffected, while cone vision is either altogether 

 absent or is found to show abnormality, which affects certain colours only. 



Note that any one of the above groups may be found to affect either one 

 or both eyes, and may involve the whole or only a limited part of the retina. 



COMPLETE BLINDNESS (group 1) may affect the whole of one 

 or both eyes, or may occur in half the visual fields only. It may be 

 limited to irregular-shaped islands or patches, or it may be found 

 associated with central or peripheral vision. The shape of the affected 

 area frequently gives a direct clue to the xjause of the condition. The 

 shape is best determined by means of the perimeter (Fig. 279). 



(a) The whole of one or both eyes is found to be blind. The disease may 

 be congenital or may be the result of inflammation affecting the posterior 

 parts of the eye (ophthalmoscope will confirm). Injury involving the whole 

 of the optic nerve trunk will also cause blindness of the corresponding eye. 



(b) The blindness involves the right or left halves of one or both retinae 

 only. The lesion in such cases involves the optic tracts. Tumours are the 

 commonest cause. 



(c) The blindness is limited to a segment of the retina when the retinal 

 vessels are affected (e.g. by embolism). The ophthalmoscope will confirm. 



(d) Blindness which affects the periphery of the retina only is due either 

 to deficiency of blood supply (as may occur in glaucoma), defective blood 

 (severe anaemia), or the presence of poisons in the blood. 



(e) If blindness affects the centre of the retina chiefly, the cause is prob- 

 ably poisoning by either tobacco, alcohol, or both. At first vision is impaired 

 for certain colours only, but the blindness quickly becomes complete if the 

 absorption of the poison continues. 



577 37 



