84 



THE MODERN SYSTEM 



the two is by the radiated expansion of one 

 order of fibres and of the blood vessels. 



Having thus come to the opening of the iris, 

 I shall call it, as before, the pupil, which is 

 not a solid body, but a passage left by the 

 iris for the rays of light to penetrate in their 

 passage to the posterior chambers of the eye. 

 The iris undergoes many changes, as to size, 

 in all animals ; and, in the Horse, as it changes 

 its form, from round to oblong, and vise versa. 

 In cats, the changes are well seen, for, in a 

 strong light it is very oblong, but perpendicular, 

 whilst it is horizontal in the Horse. This 

 shews the object of nature at once, for a cat 

 has necessity of seeing upwards and down- 

 wards for securing its prey, not sideways 

 where its powers would be lost. Ourselves 

 and the Horse have occasion to see every 

 way ; so that, though the pupil becomes hori- 

 zontal, it is never very narrow : there is also 

 a peculiar structure in the Horse's eye, which 

 seems to have the power of keeping out light, 

 occasioned by four glandular bodies, two 

 placed at the lower edge of the upper margin 

 of the iris, and two at the lower margin. 

 These are not seen in a weak light, when the 

 pupil is large ; and seldom in the dead subject: 

 they are seen best in prominent eyes, under a 

 strong light ; they are black, and covered with 

 the nigrum pigmentum (or black paint), 

 which in the dead eye becomes a mucous. 

 These bodies may almost be called an internal 

 eye-lid, and united to the iris they complete 

 the curtain. 



We will now endeavour to explain the 

 functions of the iris, which may be easily seen, 

 by observing the effect of different degrees of 

 light on the iris, light being the stimulus, and, 

 through the nervous energy, the iris becomes af- 

 fected ; the pupil being passive, the iris expand- 



ing, makes the pupil much smaller ; on the 

 other hand, if the iris contract, the pupil ap- 

 pears so much larger. The iris is possessed 

 of abundance of nerves and muscular fibres ; 

 for though this is not perceptible to the naked 

 eye, our reason demonstrates them, and we 

 must never suffer the superior light of oiu- 

 reason to be darkened by the imperfection of 

 our senses. The muscular fibres of the iris 

 are of two orders, the one appearing a kind of 

 sphincter (signifying to shut up) at the inner 

 margin, and, being excited by a strong light, 

 it contracts at the same moment ; the trans- 

 verse order relax and elongate, and thus the 

 pupil is diminished : the stimulus of light 

 being removed, the transverse preponderate, 

 diminishing the iris, and, consequently, en- 

 larging the pupil. It would appear that the 

 circular or sphincter order require a very 

 strong stimulus to be able to overcome the 

 constant superabounding power of the trans- 

 verse order of fibres, the varied action of the 

 pupil taking on different forms, according to 

 the strength of light ; in a weak light becoming 

 circular, occasioned by the transverse fibres 

 being in full play ; but, as in a strong light 

 the pupil becomes oblong, it would appear by 

 this, that the transverse fibres have not an 

 equal power of relaxation, only at the top and 

 bottom ; for, at the corners, they scarcely 

 relax at all. There is considerable sympathy 

 between the iris and the retina, as in gutta 

 serena, or glass-eyed, as it is termed ; that is, 

 when the optic nerve has lost its sensibility, 

 the iris has likewise. 



THE OUTER COATS OF THE EVE. 



In considering the outer coats of the eye, 

 we commence with the sclerotic, which is 

 composed of tendinous material, very dense. 



