506 PHYSIOLOGY 



rangement and the acute pain response, which even the smallest foreign body 

 can initiate, obviously has for its object the protection of this important 

 surface from injury. The pain impulses are conveyed to the brain either 

 via the perichoroidal nerve plexus and the long ciliary nerves to the nasal 

 branch of the 1st division of the 5th nerve, or from the plexus by the short 

 ciliary nerves to the ciliary ganglion, and from this through the radix 

 longa to the nasal, nerve. In either case the nerves appear to have their 

 cell station in the Gasserian ganglion. 



THE SCLERA, which forms the tough shell of the eyeball, consists of 

 three layers : (Da thin layer of endothelium in contact with the capsule 

 of Tenon; (2) numerous interlacing bundles of white fibrous connective 

 tissue ; (3) a layer of flat endothe'ial cells and a network of fine pigmented 

 connective tissue cells, forming the lamina fusca. 



Beside the optic nerve the sclera is perforated by the short and long 

 ciliary nerves and by the ciliary arteries. The four venae vorticosae leave 

 it at the equator. At the corneo-scleral junction the two structures are 

 continuous. A space is left however which forms a ring round the cornea. 

 This is called the canal of Schlemm ; it communicates with the anterior 

 chamber through the spaces of Fontana and also with the scleral veins. 

 The presence of these canals renders the sclero-cornea weak and therefore 

 liable to be ruptured by violence. 



THE CHOROID forms the vascular and pigmentary lining of the eye. 

 It intervenes between the sclera and the retina. Histologically it consists 

 of three layers: (1) the lamina supra-choroidea, which is similar in its 

 structure to the lamina fusca of the sclera : (2) the lamina proprja which 

 consists of connective tissue, richly supplied with blood vessels, capillaries, 

 veins, and nerves: (3) the basilar membrane of Bruch. This is a thin 

 transparent structureless layer like that of Descemet in the cornea. A highly 

 reflecting surface, called the Tapetum, is present in certain animals. This 

 is formed by a layer of iridescent cells in the lamina propria. 



THE CILIARY BODY connects the choroid to the iris. It con- 

 of three parts : (1} the ciliary muscle, the function of which is to cause 

 the accommodation of the lens; (2) the ciliary glands which secrete t In- 

 aqueous humour ; and (:i) the orbiculus which is the part of 1 lie ciliary body 

 connecting it with the choroid. The ciliary bodies are covered by a thin 

 pigmented layer which is a continuation ol the retina. This also covers 

 the posterior surface of the iris and ends there. 



THE IRIS consists of three layers: (I) the endothelium continuous with 

 that on the posterior surface of the cornea : (2) the stroina of the iris, which 

 consists of connective tissue (especially elastic fibres), two thin sheets of 

 muscle, some pigment cells, vessels and nerves; (3) the pigmented layer 

 continuous with the retina. 



It should be noted that the posterior elastic lamina of Descemet in the 

 cornea, after its division into three parts, forms bv its posterior portion 

 the ligamentuin pectinatum iridis, by which the iris irains attachment to 

 the sclero cnrneal junction. 



