NERVE 143 



forwards over the ciliary processes and over the back of 

 the iris. 



The blood-vessels of the retina enter in the middle of the 

 optic nerve, and run out and branch in the anterior layer of 

 the retina. 



The interior of the eye may be examined by the 

 Ophthalmoscope, either by the direct or by the indirect method 

 (fig. 64). By the direct method light is reflected from a 

 small mirror into the eyes, and the observer examines the 

 fundus directly through a hole in the middle of the mirror 

 (fig. 64, A). By the indirect method he reflects light from 

 a slightly concave mirror into the eye, and then inserts 

 between the eye and the mirror a biconvex lens so that an 

 image of the fundus is formed, and this is examined (fig. 

 64, B) {Practical Physiology). 



The fluid which distends the eyeball may be considered as 

 a form of lymph. 



It is derived by exudation from the vessels in the ciliary 

 processes. Some passes back into the vitreous humour, but 

 the greater quantity passes forward into the anterior chamber, 

 from which it is drained away in the lymph spaces in the 

 anterior part of the sclerotic called the spaces of Fontana, 

 which open into the canal of Schlemm. When these spaces 

 become obstructed, the fluid tends to accumulate, and the 

 pressure in the eyeball rises and it feels hard to the touch. 

 This condition of glaucoma leads to disturbances in the 

 nutrition of the eyeball. 



The pressure in the eyeball varies with the arterial 

 pressure, and it is also influenced by any obstruction to the 

 flow of blood in the veins from the head so that in cases of 

 intracranial tumour the vessels of the retina are apt to 

 become congested. 



O" 



C. Physiology. 



The study of vision may be taken up in the following 

 order : — 



1. The mode of formation of pictures on the nerve structures 

 (retina) of the eye. 



