384 SENSITIVE SYSTEM. 



nos, in whose eyes there is the same deficiency. This surface 

 has hkewise a division into two parts by a prominent elliptical 

 fold in the middle: the larger one exhibits numerous convergent 

 strise which run in radii from the outer margin, where they look 

 almost like continuations of the ciliary processes, and terminate 

 in this prominent fold ; the inner and lesser division is a phme 

 oval surface, apparently without striaB, perforated in the centre 

 by the pupil. 



Margins. — J'he greater or ciliary margin of the iris is em- 

 braced by the ciliary circle, and has a broad attachment to the 

 choroid coat besides, through the continuation of the pigmentum 

 nigrum. The lesser or pupillary margin, rendered black and 

 prominent by an edging from the uvea, and hanging loose and 

 floating in the aqueous humour, constitutes the boundary line of 

 the pupil, and gives attachment to the corpora nigra. 



The pupil, then, is nothing more than a hole in the iiis, oblong 

 or elliptical in the same direction as the cornea is, whose diame- 

 ter varies with the intensity of the light to which the eye is ex- 

 posed. There are animals, however, in which the pupil does not 

 ordinarily correspond in shape with the cornea, in consequence of 

 its altering its figure as well as its magnitude under sucli circum- 

 stances : e.g. in the cat, the cornea is circular, but the pupil dur- 

 ing the daylight is elliptical in the vertical direction, though at 

 night it becomes circular ; and, indeed, the pupil of the horse, 

 widely dilated from the effects of amaurosis or the influence of 

 belladonna, likewise assumes the circular figure. 



Structure. — All that we know about the structure of the iris, 

 is, that it is apparently a fibrous membrane, divisible into two 

 layers, provided abundantly with bloodvessels and nerves, and 

 thickly coated behind with uvea : the division of it cannot be 

 carried through the pupillary margin, there its layers becoming 

 inseparable. Its fibres are believed by some to be muscular, the 

 quick and free motions of the part forming the principal grounds 

 of their argument; for the fibres (even by their own admission) 

 are too minute to admit of anatomical demonstration. Professor 

 Coleman found them well marked in the eye of a greyhound that 

 had been long inured to cunning coursing in a rabbit warren. 

 Not only do these fibres elude all anatomical test of their nature, 

 they are insensible to the common mechanical and chemical 

 excitants of other muscles, and even to galvanism. — The arteries 

 of the iris come from the long ciliary, and are arranged so as to 

 form two circuli arterosi upon its anterior smface, corresponding 

 to the two prominent pliccB thereon : from these, minute branches 

 are detached, which are so numerous, that they of themselves 

 appear to constitute the basis or essential part of its texture. 



