ORGAN OF VISION. 



215 



Fig. 85. 



near a convex glass, for the purpose of diminishing the surface of the 

 lens accessible to the rays of light, and thus of obviating spherical 

 aberration. The interior of the tube and of the diaphragm is coloured 

 black, to absorb the oblique rays, which are not inservient to vision ; 

 and thus to prevent them from causing confusion. This arrangement 

 is nearly a counterpart of that which exists in the eye. The tube of 

 the instrument is represented by three membranes in superposition, 

 the sclerotic, choroid, and retina; the last receiving the impression of 

 light. Within, are four refracting bodies, situate one behind the other ; 

 and intended to bring the rays of light to determinate foci, the cornea, 

 aqueous humour, crystalline lens, and vitreous humour. Lastly, in the 

 interior of the eye, near the anterior surface of the crystalline, is a 

 diaphragm the iris, having an aperture in its centre the pupil. 

 These different parts demand a more detailed notice. 



1. Coats of the Eye. Before describing the coats of the eye it may 

 be remarked, that the eyeball is invested with 

 a membranous tunic, which separates it from 

 the other structures of the orbit ; and forms 

 a smooth, hollow surface by which its motions 

 are facilitated. This investment has been 

 variously called, cellular capsule of the eye, 

 ocular capsule, tunica vaginalis oculi, and 

 submuscular fascia. 



The sclerotic is the outermost proper coat. 

 It is that which gives shape to the organ, and 

 which constitutes the white of the eye. It is 



of a dense, resisting, fibrous nature, belonging Side View of the same Eye> as 

 to what M. Chaussier calls alougineous tissue. 

 Behind, it is penetrated by the optic nerve ; 

 and before, the cornea is dovetailed into it. 

 It has, by some anatomists, been considered 

 a prolongation of the dura mater, accompany- 

 ing the optic nerve ; whilst the 

 choroid has been regarded as 

 an extension of the pia mater ; 

 and the retina of the pulp of 

 the nerve. The sclerotic is the 

 place of insertion for the various 

 muscles that move the eyeball, 

 and is manifestly intended for 

 the protection of the internal 

 parts of the organ. 



Immediately within the scle- 

 rotica, and feebly united with it 



in Fig. 84, showing that the 

 Cilia of the Upper Lid are con- 

 cave upwards, and those of the 

 Lower Lid concave down- 

 wards. The general Convexity 

 of the Eyeball is seen. 



Fig. 86. 



Choroid Coat of the Eye. 



Curved lines marking the arrangement of venae 

 vorticosce. 2, 2. Ciliary nerves. 3. A long ciliary ar- 



by vessels, nerves, and areolar 

 tissue, 1 is the choroid coat; a 



n, ,1 i , vui iiuusue. -&.-&, Binary nerves, o. A mug umaiy ctr- 



SOlt, tiim, VaSCUlar, and nerVOUS tery and nerve. 4. Ciliary ligament. 5. Iris. 6. Pupil. 



1 In the situation of this areolar tissue, Arnold describes a serous membrane, Spinnwebenhaut 

 JLrachnoidea oculi, Lamina fusca sderotica Arnold uber das Auge, Tab. iii., Fig. 2, and Weber's 

 Hildebrandt's Haridbuch der Anatomic, iv. 68, Braunschweig, 1832. 



