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A MANUAL OF VETERINARY PHYSIOLOGY 



convex at another. This alteration in shape occurs through the 

 ready manner in which the lens, by its elasticity, yields to the 

 pressure exercised on it through its capsule, so that if the tension 

 of the capsule be relaxed the lens bulges, or if the tension be 

 increased it flattens. In this way the eye is focussed or accom- 

 modated to various distances, a subject which will be dealt with 

 presently. 



The Iris is a curtain with a hole in the centre, called the pupil. 

 The shape of the pupil varies in different animals. In the dog 

 it is circular ; in the horse, sheep, ox, and cat elliptical. In the 

 latter animal the elliptical slit is placed vertically ; in the others 

 horizontally. Berlin, whose work will be referred to presently, 

 regarded the horizontal pupil as increasing the distinctness of 

 the retinal image. Both in the horse and cat, according to this 



observer, the direction 

 of the pupil corre- 

 sponds to the least 

 curved meridian of the 

 & cornea. The iris is 

 1 mainly a collection of 

 bloodvessels and mus- 

 cular fibres, the whole 

 £ being heavily coated 

 with a brown pigment 

 in the horse, though 

 occasionally this is 

 wanting, giving it a 

 bluish - white streaky 

 appearance, as in the 

 so-called ' wall-eyed ' horse. In the ox and dog the iris is 

 a brighter brown than in the horse, while in the sheep it is 

 brownish-yellow. The muscular fibres of the iris are of the 

 unstriped variety, and disposed in a circular and radiating 

 manner. A contraction of the circular muscle contracts the 

 pupillary opening ; a contraction of the radiating fibres dilates 

 it. On bleaching the iris of the horse the disposition of its 

 muscular fibres can be easily studied. Fig. 159 shows the peculiar 

 wrinkles in the curtain and pleats in the iris above and below 

 the pupil, caused by the radiating fibres. Immediately sur- 

 rounding the pupillary opening is a circular layer of muscular 

 fibre. The wrinkles and pleats in the iris of the horse are con- 

 fined to its anterior face ; the surface next the lens is perfectly 

 smooth. Langley and Anderson, from their observations on 

 the cat, dog, and rabbit, proved that a dilator muscle to the 

 iris exists. This question was for a long time in dispute. It 

 is now accepted that dilatation of the pupil is due to the 



159. — Bleached Iris of the Horse. 



1, Pupillary opening ; 2, corpus nigrum ; 3, iris 

 showing wrinkles and pleats in its structure ; 

 4, circular muscle surrounding the pupillary 

 opening; 5, Ligamentum pectinatum. 



