926 TBE SENSORY APPARATUSES. 



The essential organ of vision, or globe of the ege, will be first described : then, 

 under the designation of the accessory portion of the cisual apparatus, v,e will 

 notice the receptacle for this globe, or orbital cavity ; the muscles that move the 

 eye ; the protective membranes, or eyelids ; the membrana nictitans, or accessor/) 

 eyelid ; and, lastly, the lachrymal apparatus, which concurs in the protection of 

 the ocular globe by the fluid it incessantly throws out upon its surface. 



Article I. — The Essential Organ of Vision, or Ocular Globe (Fig. 502). 



Preparation. — The eye is removed from the orbit, and freed from the muscles and fat on its 

 posterior surface. Its membranes may then be dissected by means of delicate instruments, and 

 an idea formed of the structure of the eye. But in order to study the form and relations of the 

 different membranes and media of the eye, the globe should be hardened in gradually con- 

 centrated solutioiLS of chromic acid. When it is so hardenrd, sections can be made in every 

 direction, at any distance from the axes, as the shell of the eye does not become deformed. It 

 it is very easy to study the organization of the eye on such preparations. 



The globe, or ball of the eye, is a spherical shell, the interior of which is filled 

 with fluid or semifluid parts, named the humours or media of the eye. The wall 

 of this shell {tunica externa) is formed of a continuous, very resisting, colourless 

 envelope, limpid and translucid in its anterior portion, which constitutes the 

 transparetit cornea ,• and white and opaque for the remainder of its extent. It is 

 known as the sclerotica. 



On the inner face of the sclerotica is a second tunic (tunica media') — the 

 choroid: a black membrane that lines the posterior face of the retina {tunica 

 interna), and which, near where the two constituent portions of the external 

 envelope unite, throws into the interior of the eye an elliptical diaphragm with 

 a large opening in its centre — the iris. Immediately behind this disc is suspended 

 or set, like a rose-brilliant, in the centre of a circular zone depending from the 

 choroid, a biconvex body — the crystalline lens, one of the media of the eye, and 

 which divides the interior of its cavity into two compartments : a posterior, very 

 large, occupied by the vitreous humour ,• and an anterior, itself divided by the 

 iris into two chambers of unequal dimensions, which contains the aqueous 

 humour. 



Viewed externally, and as a whole, the organ resulting from the union of all 

 these parts represents a globular body, the anterior region of which corresponds 

 to the cornea, and is more convex than the other points : a circumstance that 

 tends to increase the antero-posterior diameter of the eye. But as this ocular 

 sphere — to which is added, in front, this segment of a smaller sphere — is sensibly 

 depressed from before to behind, it results that the other two principal diametere 

 — the vertical and transversal — offer about the same dimensions as the first ; 

 Girard has even stated that the latter is the least. With an eye hardened by 

 chromic acid, we have found that the transverse diameter measured 0*036 m., and 

 the vertical 0-040 m. (1-417 X 1-575). 



Two paragraphs will be devoted to the description of the constituent parts of 

 the globe — one for the membranes, the other for the media. 



The membranes (or tunics) of the eye are of three kinds. Two are exclusively 

 fibrous, and form the framework ; these are the sclerotica and cornea. The second 

 is characterized by the association in its interior of the contractile element and 

 of vessels ; and the third is represented by the retina, which is nervous in 

 structure. 



