Myopia 87 



the book ; then, that he may have binocular vision, the internal, the 

 superior, and inferior recti must be in constant contraction. This 

 causes the posterior, and unsupported, part of the feeble globe to 

 bulge, so that its length becomes increased. The strained and stoop- 

 ing position of the head compresses the cervical veins, and hinders 

 the return of blood from the eye-ball, which is already in a condition 

 of too great physiological activity, and in which there is also excessive 

 intra-ocular secretion. 



That there is increased intra-ocular pressure is evidenced by the 

 fact that the choroid becomes so much flattened and thinned in the 

 neighbourhood of the optic nerve that a myopic crescent of the sclerotic 

 can be detected by the ophthalmoscope. 



The treatment is first preventive : 



' Not with blinded eyesight poring over miserable books. ' 



Subsequently it demands the careful adjustment of concave glasses, so 

 that the rays of light may be rendered more divergent before they reach 

 the cornea. 



The iris is the coloured circular curtain which floats in the 

 aqueous humour, in front of the lens, its central opening being the 

 pupil. By its circumference it is attached to the junction of the 

 sclerotic and cornea, and is continuous behind with the ciliary 

 processes. 



Structure. The iris is composed of delicate connective tissue, 

 the filaments being chiefly arranged in a radiating manner. Towards 

 the anterior part are a group of coloured cells, and behind is a dark 

 pigmentary layer, the uvea (uva, a bunch of grapes], 



The posterior surface of the iris moves upon the front of the lens, 

 the narrow space between it and the periphery constituting the 

 posterior chamber of the aqueous. The anterior chamber is the 

 interval in front of the iris, and, like the posterior chamber, is virtually 

 a large lymph space. 



The muscular tissue consists of a sphincter of plain fibres around 

 the pupil, and of a radiating series which extend through the width 

 of the iris. The sphincter set are governed by the third nerve, the 

 dilator fibres being under the influence of the sympathetic. 



Up to the seventh month of foetal life a pupillary membrane occu- 

 pied the central opening of the iris. Its arteries were derived from 

 the arteria centralis retinae and from those of the iris. In the eighth 

 month the membrane begins to clear away from the centre, but occa- 

 sionally it persists after birth. 



The vessels of the iris. The long ciliary one running on 

 either side of the optic nerve eventually send an arterial circle 

 around the attached border of the iris, where they anastomose with 

 the short ciliary. Both sets then send twigs through the iris to make 

 an anastomotic circle round the pupillary border. 



