THE EYE 209 



In repair of inflammatory conditions of the cornea, the 

 newly formed connective tissue, not being specialised, may 

 cause opacities, which vary in degree from nebulae to leucoma. 

 Iridectoiny may be of value in certain cases, as it offers a larger 

 aperture for the transmission of light. 



Internal to the sclera is the Chorioid, which supports the 

 ciliary vessels and contains numerous pigment cells. Anteriorly, 

 the chorioid is continuous with the Ciliary Body and the Iris. 

 The iris separates the anterior from the posterior chamber of 

 the eye. Its peripheral attachment is continuous behind with 

 the ciliary body and in front with the deepest layer of the cornea, 

 which it meets at an acute angle. A circular channel, the sinus 

 venosus sclerse (of Schlemm), is placed in the corneo-scleral 

 junction immediately opposite the angle between the iris and 

 the cornea (Fig. 61). The sinus is surrounded by numerous 

 lymph spaces, through which the aqueous humour and the 

 lymph from the cornea communicate with it and indirectly with 

 the ciliary veins. Obliteration of this angle by the formation of 

 adhesions prevents the proper drainage from the aqueous 

 humour, and causes increased intra-ocular tension and glaucoma. 

 The iris is richly supplied with blood, and the ciliary arteries 

 form anastomosing circles at its free and attached margins. 



The optic nerve pierces the retina at a point a little medial to the posterior 

 pole of the eyeball. The papilla of the optic nerve (optic disc) is the physio- 

 logical " blind spot " of the retina. With the aid of the ophthalmoscope, 

 the central artery of the retina can be seen on the papilla breaking up into its 

 terminal branches. In all mammals which have binocular vision, the point 

 on the retina through which the antero-posterior axis of the eyeball passes 

 is highly specialised, and is known as the macula lutea. 



The Vitreous Body, which occupies the greater part of the posterior chamber 

 of the eye, is surrounded by the hyaloid membrane. In front, the membrane 

 is attached to the ciliary body, and divides to form the suspensory ligament 

 which encloses the Crystalline Lens. When the ciliary muscle, which is con- 

 tained in the ciliary body, contracts, the suspensory ligament is drawn 

 forwards, and the pressure which it exerts on the anterior surface of the lens 

 is diminished. As a result, this surface becomes more convex, and diverging 

 rays of light become focussed on the retina. 



The diameter of the crystalline lens is one-third of an inch, and this 

 measurement must be remembered in making the incision in the corneo- 

 scleral junction for its removal. 



THE AUDITORY APPARATUS. 



Development. The outer cartilaginous portion of the ex- 

 ternal acoustic meatus is derived from the dorsal part of the 



14 



