340 MICROSCOPIC ANATOMY OF THE ORGANS. 



cells are nucleated, and include in their protoplasm many 

 free fat globules. They are derived probably from leucocytes 

 which have wandered into the lumen of the alveolus. Some 

 authors regard them MS gland cells which have undergone 

 fatty change. 



2. VISUAL ORGAN. 



The true organ of vision consists of the eyeball (bulbus ocu\i) 

 and the optic nerve. Besides these there are protective struct- 

 ures, the eyelids and the lachrymal apparatus. 



(a) Eyeball. 



In the walls of the eyeball there are three layers : 



(1) Tunica externa seu fibrosa, which consists of the opaque 

 sclera and the transparent anterior part, the cornea. 



(2) Tunica media seu vasculosa, which is made up of the 

 choroidea, the ciliary body, and the iris. 



(3) Tunica interna, which consists of the retina. 



The eyeball contains in its interior the aqueous and vitreous 

 humors and the lens crystallina. 



(1) Tunica Externa. 



The cornea (Fig. 259) is a membrane varying in thickness 

 from 0.8 to 1.1 mm. In it can be made out five layers, which 

 from in front backward are as follows : 



(1) The anterior epithelial layer (cornea! epithelium) ; 



(2) The lamina elastica anterior ; 



(3) The substantia propria corneas ; 



(4) The lamina, elastica posterior ; 



(5) The posterior epithelial layer (corneal endotheliumj. 

 (1) The most superficial sheath consists of five to eight 



layers of epithelial cells; the deepest of which are cylindrical. 

 These pass over into lower polygonal cells, which at the sur- 

 face become flat, but are always nucleated. Regeneration tab > 

 place in the basal cylindrical cells, in which karyokinetic 

 figures are found not infrequently. The cells are bound 

 together by intercellular bridges, as in the skin. The lower 



