388 SENSITIVE SYSTEM. 



part which is within the axis of vision. In a perfectly fresh eye, 

 the tapetum is bright, and its hmits are distinctly marked ; in- 

 deed, with pains, it may be stripped cleanly off the sclerotica, 

 and even without staining the fingers in so doing ; but putrefac- 

 tion destroys its verdure and consistency, and converts the entire 

 pigment into a thin, sooty, semi-fluid matter, readily diffusible 

 in water, and which leaves a dusky stain upon the sclerotica and 

 other parts in contact with it. The pigmentum and tapetum 

 (apparently the same substance, differing only in colour) are 

 supposed to be a peculiar secretion from the vessels of the cho- 

 roides — a secretion of a mucous nature — only that it is con- 

 tained within a very fine, soft, and delicate cellular tissue by 

 which it is held together. Mr. Hunter compared it to the rete 

 mucosum : and there would appear not only to be a similarity in 

 composition, but also some mysterious relationship existing be- 

 tween them in regard to their formation ; for, in Albinos, in 

 purely white and cream-coloured horses, in white rabbits, ferrets, 

 &c. in whose eyes the pigment is deficient, the rete mucosum 

 is also wanting : this accounts for the delicate whiteness of their 

 skins, and for the redness of their eyes, in which the blood (cir- 

 culating in the vessels of the choroid) is seen through the pupil, 

 unobscured by colouring matter. 



Ciliary Processes. — Leaving the ciliary circle, the choroides 

 makes a sudden inflection inward behind the iris, and then turns 

 intensely black. This inflection considerably contracting the 

 space the tunic was formerly spread over, there results a super- 

 abundance of substance, which we find to be disposed of in 

 puckers or plaits, arranged in parallel lines, like-radii, around 

 the circumference of the crystalline lens : regarding the doubling 

 and plaits together as a whole, the part is named the corpus 

 ciliare ; but the plaits by themselves are called the processus 

 ciliares. These processes repose, posteriorly, upon the soft bed 

 of the vitreous humour, whereupon they make counter-impres- 

 sions, the membramda corona ciliaris being alone interposed. 

 The corpus ciliare is coated with pigment of the deepest dye — 

 the interstices of the processes in particular are thickly bespread 

 with it : this explains the remarkable blackness of this part, and 

 accounts for the radiated circle of black stria marked upon the 

 surface of the membranula ciliaris, after the processes have been 

 stripped off. In order to examine the structure and connexions 

 of the corpus ciliare, the pigment should be washed oft. It will 

 then be found to be continuous at the ciliary circle with the 

 choroid, to possess the same vascular basis as that tunic, and, in 

 fact, in nowise to differ from the choroid but in its disposition 

 and arrangement. The internal part of the choroides near the 



