G20 ORGANS OF THE SENSES. 



The muscular fibre is involuntary, and consists of circular and radiating fibres. 

 The circular fibres (sphincter of the pupil) surround the margin of the pupil on 

 the posterior surface of the iris, like a sphincter, forming a narrow band, about 

 one-thirtieth of an inch in width ; those near the free margin being closely aggre- 

 gated ; those more external are separated somewhat, and form less complete circles. 

 The radiating fibres (dilator of the pupil) converge from the circumference towards 

 the centre, and blend with the circular fibres near the margin of the pupil. The 

 circular fibres contract the pupil, the radiating fibres dilate it. 



The pigment cells are found in the stroma of the iris, and also as a distinct layer 

 on its anterior and posterior surfaces. In the stroma, the cells are ramified, and 

 contain yellow or brown pigment, according to the color of the eye. On the front 

 of the iris, there is a single layer of oval or rounded cells, with branching offsets. 

 On the back of the iris, there are several layers of small, round cells, filled with 

 dark pigment. *This layer is continuous with the pigmentary covering of the 

 ciliary processes. 



The arteries of the iris are derived from the long and anterior ciliary, and from 

 the vessels of the ciliary processes. 



Membranapupillaris. In the foetus, the pupil is closed by a delicate, transparent 

 vascular membrane, the membrana pupillaris, which divides the space in which the 

 iris is suspended into two distinct chambers. This membrane contains numerous 

 minute vessels continued from the margin of the iris to those on the front part of 

 the capsule of the lens. These vessels have a looped arrangement, converging 

 towards each other without anastomosing. Between the seventh and eighth month, 

 this membrane begins to disappear, by its gradual absorption from the centre 

 towards the circumference, and at birth only a few fragments remain. Sometimes 

 it remains permanent, and produces blindness. 



The Ciliary ligament is a narrow ring of circular fibres, about one-fortieth of 

 an inch thick, and of a whitish color, which serves to connect the external and 

 middle tunics of the eye. It is placed round the circumference of the iris, at its 

 point of connection with the external layer of the choroid, the cornea, and sclerotic. 

 Its component fibres are delicate, and resemble those of elastic tissue. At its point 

 of connection with the sclerotic a minute canal is situated between the two, called 

 the sinus circularis iridis. 



The Ciliary muscle (Bowman) consists of unstriped fibres ; it forms a grayish, 

 semi-transparent, circular band, about one-eighth of an inch broad, on the outer 

 surface of the forepart of the choroid. It is thickest in front, and gradually 

 becomes thinner behind. Its fibres are soft, of a yellowish- white color, longitu- 

 dinal in direction, and arise at the point of junction of the cornea and sclerotic. 

 Passing backwards, they are attached to the choroid, in front of the retina, and 

 correspond by their inner surface to the plicated part of the former membrane. 

 Mr. Bowman supposes that this muscle is so placed as to advance the lens, by 

 exercising compression on the vitreous body, and by drawing the ciliary processes 

 towards the line of junction of the sclerotic and cornea, and by this means to 

 adjust the eye to the vision of near objects. 



THE KETINA. 



The EETINA may be exposed by carefully removing the choroid from its external 

 surface. It is a delicate nervous membrane, upon the surface of which the images 

 of external objects are received. Its outer surface is in contact with the pigmentary 

 layer of the choroid ; its inner surface, with the vitreous body. Behind, it is con- 

 tinuous with the optic nerve ; it gradually diminishes in thickness from behind 

 forwards ; and, in front, extends nearly as far forwards as the ciliary ligament, 

 where it terminates by a jagged margin, the ora serrata. It is soft, and semi- 

 transparent, in the fresh state ; but soon becomes clouded, opaque, and of a pinkish 

 tint. Exactly in the centre of the posterior part of the retina, and at a point 

 corresponding to the axis of the eye, in which the sense of vision is most perfect, 

 is a round, elevated, yellowish spot, called, after its discoverer, limlus luteus, 



