THE SENSES. 205 



refracting structure of the eye. The structure of the lens is very complex. 

 It consists essentially of fibres united side by side to each other, and 

 arranged together in very numerous laminae, which are so placed upon 

 one another, that when hardened in spirit the lens splits into three por- 

 tions in the form of sectors, each of which is composed of superimposed 

 concentric laminae. The lens increases in density and, consequently, in 

 power of refraction, from without inward; the 

 central part, usually termed the nucleus, being 

 the most dense. 



The vitreous humor constitutes nearly four- 

 fifths of the whole globe of the eye. It fills 

 up the space between the retina and the lens, 

 and its soft jelly-like substance consists essen- 

 tially of numerous layers, formed of delicate, 

 simple membrane, the spaces between which 



are filled with a Watery, pellucid fluid. Its FIG. 374.-Laminated structure 



principal use appears to be that of giving the f a f m | iSeSSS^af ? harden* 

 proper distension to the globe of the eye, and ^i^l^t^SS 

 of keeping the surface of the retina at a proper cessive external layers. A. CAT- 

 distance from the lens. 



Action of the Iris. The iris is a vertically -placed membranous dia- 

 phragm, provided with a central aperture, the pupil, for the transmission 

 of light. It is composed of plain muscular fibres imbedded in ordinary 

 fibro-cellular or connective tissue. The muscular fibres have a direction, 

 for the most part, radiating from the circumference toward the pupil; but 

 as they approach the pupillary margin, they assume a circular direction, 

 and at the very edge form a complete ring. By the contraction of the 

 radiating fibres (dilator pupillse) the size of the pupil is enlarged: by the 

 contraction of the circular ones (sphincter pupillae), it is diminished. The 

 object effected by the movements of the iris, is the regulation of the 

 quantity of light transmitted to the retina. The posterior surface of the 

 iris is coated with a layer of dark pigment, so that no rays of light can 

 pass to the retina, except such as are admitted through the aperture of 

 the pupil. 



This iris is very richly supplied with nerves and blood-vessels. Its 

 circular muscular fibres are supplied by the third (by the short ciliary 

 branches of the ophthalmic ganglion), and its radiating fibres, by the sym- 

 pathetic and fifth cranial nerve (by the long ciliary branches of the nasal 

 nerve). 



Contraction of the pupil occurs under Hie folio wing circumstances: (1) 

 On exposure of the eye to a bright light; (2) when the eye is focussed for 

 near objects; (3) when the eyes converge to look at a near object; (4) on 

 the local application of eserine (active principle of Calabar bean); (5) on 

 the administration internally of opium, aconite, and in the early stages 



