586 ORGANS OF SENSE. 



circumference towards the centre, and blend with the circular fibres near the 

 margin of the pupil. 



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 rami- 

 fied, 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. 



Membrana pupillaris. In the foetus, the pupil is closed by a delicate, transpa- 

 rent, vascular membrane, the mernbrana pupillaris, which divides the space in 

 which the iris is suspended into two distinct chambers. This membrane con- 

 tains 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, the membrane begins to disappear, by its gradual 

 absorption from the centre towards the circumference, and at birth only a few 

 fragments remain. It is said sometimes to remain permanent, and produce 

 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 around 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 this 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, 

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

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

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

 tudinal in direction, and arise at the point of junction of the cornea and scle- 

 rotic. 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 Retina 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 continuous 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 semitransparent, 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, the yellow spot or limbus luteus, of Sommerring ; having a central 

 depression at its summit, the/owa centralis. The retina in the situation of the 

 fovea centralis is exceedingly thin; so much so, that the dark color of the 

 choroid is distinctly seen through it; so that it presents more the appearance 

 of a foramen, and hence the name 'foramen of Sommerring' at first given to it. 

 It exists only in man, the quadrumana, and some saurian reptiles. Its use is 

 unknown. About ^ of an inch to the inner side of the yellow spot, is the 



