THE STRUCTUKE OF THE EYEBALL 



505 



the layers between are a gradual transition from one type to the other. 

 (2) The anterior elastic lamina of Bowman. This is not true elastic 

 tissue, but a layer of modified substantia propria. (3) Substantia propria 

 which consists of a special type of fibrous connective tissue. The fibres 

 are arranged in parallel rows to form laminse, and the laminae are built 

 one above the other, leaving cell spaces or lacunse between. The fibres of 

 each lamina are cemented together by an amorphous substance of nearly 

 the same optical density so that the laminse form one homogeneous structure. 

 It is on this arrangement that the transparency of the cornea depends. 

 If an excised eye be squeezed so as to produce a high intraocular tension, 

 the cornea is seen to become partially opaque. This is caused by the 

 tension in the corneal fibres making them become doubly-refracting in a 

 similar manner to that set up by the contraction in a striated muscle fibre. 

 But owing to this double refraction the laminse of the cornea cease to 

 form one homogeneous structure, and therefore opacity is the result. 

 Within the lacunae are to be found 

 the corneal corpuscles, which are flat 

 nucleated star-shaped cells. (4) The 

 posterior elastic lamina of Descemet. 

 This is a clear structureless membrane 

 which splits at its periphery into three 

 layers. The first enters the sclera, the 

 second gives attachment to the ciliary 

 muscle, while the third enters the iris 

 as the ligamentum pectinatum and 

 gives attachment to it ; the intervals 

 between its fibres are called the spaces 

 of Fontana. (5) A layer of endothe- 

 lium. This consists of a single layer of 

 flat nucleated cells which line the 

 spaces of Fontana and the anterior 

 surface of the iris. 



The cornea is nourished during 

 health by a flow through the cell spaces 

 of lymph which comes from the peri- 

 pheral vessels. During its development 

 and when diseased it is supplied by 

 capillaries which run in from its edge. 

 Its sensory nerve supply is extremely 

 rich, but pain end organs alone appear 

 to be present. 



IHistologically the sensory nerve 

 filaments are found to ramify actually 

 in the surface layers of the stratified 

 epithelium, a condition not found in 

 any other part of the body. This ar- 



FIG. 253. Nerve supply to the eyeball. 



(After FOSTER.) 



l.g, lenticular ganglion with its three 

 roots, viz. : r.b, radix brevis or short 

 root ; r.l, radix longus or long root ; 

 sym, sympathetic root ; V. opth. oph- 

 thalmic division of V nerve ; /// oc.m, 

 oculo-mptor nerve ; //, optic nerve ; 

 l.c, long ciliary nerves ; s.c, short ciliary 

 nerves. 



