612 PHYSIOLOGY 



wall of the eye is a strong muscular ring, composed of meridional fibres, which 

 run from the cartilaginous ring to be inserted into the ciliary processes or corpus 

 ciliare, which is closely attached to the equator of the lens. 



When this muscle contracts it pulls back the whole anterior wall of the 

 eye together with the lens, approximating it to the retina. This movement 

 is of necessity accompanied by a rise of ocular pressure, but room for the dis- 

 placed fluid is found by a bulging of the walls of the eyeball at their thinnest 

 part, i.e. just behind the cartilaginous ring, so that there is an actual diminu- 

 tion of the distance between the lens and the retina. 



In every class of animals, except in the cephalopod and in birds, species are 

 found which possess no power of accommodation at all, or only to a very slight 

 extent. This is the case in frogs, alligators, vipers, and in many rodents. Many 

 of these animals are distinguished by nocturnal habits, and in daylight their 

 pupils may be constricted to such an extent as to render accommodation 

 unnecessary. In many of them, too, the exact form of an object is not so 

 important as the power to follow its movements. In such cases the movement 

 of the extrinsic ocular muscles or of the head are more important than the 

 exact focusing of the object on the retina. 



THE FUNCTIONS OF THE IRIS 



. The iris is the forward prolongation of the pigmented choroid 

 coat. It is covered anteriorly by a layer of epithelium continuous 

 with Decemet's epithelium, and behind by a thick layer of pigmented 

 epithelium which is prolonged forwards from the retina. It is 

 composed of delicate connective tissue, attached at its circumference 

 to the fibres of the ligamentum pectinatum, and contains two sets of 

 unstriated muscular fibres. The one set, the sphincter pupillm, is 

 composed of fibres which run a circular course around the margin of 

 the pupil. The other set, the dilatator pupillce, forms a flattened 

 layer of radiating fibres, which lie close to the posterior surface and 

 extend from the attachment of the iris nearly to the rim of the pupil. 



The pigment in the iris and choroid serves the same purpose as the 

 blackened lining, which is supplied to every optical instrument, in 

 preventing dispersion of the incident light, and therefore preventing 

 any light falling on the retina except those rays which pass through 

 the pupil and refractive surfaces of the eye. The pigment in the iris 

 has an additional importance in that it enables this organ to act as a 

 diaphragm. It not only shields the retina, the sensory apparatus of 

 the eye, from the effects of any excess of illumination, but, by stopping 

 out the rays of light passing through the periphery of the lens, it 

 diminishes spherical aberration and enables a clear image of external 

 objects to be formed on the back of the eyeball. The diameter of 

 the pupil is continually varying, according to the amount of light 

 falling into the eye and the condition of the mechanism of accom- 

 modation. 



Contraction of the pupil occurs under the following circumstances : 

 (1) When light falls on the retina. This movement, which is 



