218 THE ORGANS OF SPECIAL SENSE 



The macula lutea is placed at the posterior pole of the eye. 

 It contains a large number of the highly specialised ganglionic 

 cells, since it is the area on which the entering light rays are 

 focused. On this account, too, it is crossed by no blood- 

 vessels, and it depends for its nutrition on transudation from 

 the surrounding areas. It lies slightly above and to the lateral 

 side of the porus opticus. 



When the porus opticus (optic disc) is being examined, it 

 must be remembered that the condition of the media the 

 cornea, the aqueous humor, the crystalline lens and the 

 vitreous body may greatly affect the field examined. Thus, 

 in astigmatism, the outline of the porus opticus is greatly dis- 

 torted, but it is the cornea and not the porus which is at fault. 



Ophthalmoscopic Examination offers a means of examining 

 not only the coats and media of the eyeball, but also the 

 condition of the peripheral circulation and blood-vessels. 

 Important information can be obtained by this means in cases 

 of early arterio-sclerosis. The loss of elasticity in the arterial 

 wall produces in the retina, as elsewhere, tortuosity of the 

 arteries, and, if the veins are examined, it will be found that 

 they are compressed at the points where they are crossed by 

 the thickened arteries, with or without peripheral engorgement, 

 depending on the degree of pressure. The veins are the more 

 readily influenced on account of the thinness of their walls 

 (vide supra). Later, the vascular obstruction and the pressure 

 on the lymph-spaces which surround the veins cause oedema 

 of the retina. 



The Nose is described on page 325. 



