898 THE ORGANS OF SENSE. 



medullated fibres which terminate in the circular and radiating muscular fibres. 

 Their exact mode of termination has not been ascertained. Other fibres from the 

 plexus terminate in a network on the anterior surface of the iris. The fibres 

 derived from the motor root of the lenticular ganglion (third nerve) supply the 

 circular fibres, while those derived from the sympathetic supply the radiating 

 fibres. 



Membrana Pupillaris. In the foetus the pupil is closed by a delicate transparent 

 vascular membrane, the membrana pupillaris, which divides the space into 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, and converge 

 toward each other without anastomosing. Between the seventh and eighth months 

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

 toward the circumference, and at birth only a few fragments remain. It is said 

 sometimes to remain permanent and produce blindness. 



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. It consists of two sets of fibres, radiating and circular. 

 The former, much the more numerous, arise at the point of junction of the cornea 

 and sclerotic, and, passing backward, are attached to the choroid opposite to the 

 ciliary processes. One bundle, according to Waldeyer, is continued backward to 

 be inserted into the sclerotic. The circular fibres are internal to the radiating ones, 

 and to some extent unconnected with them, and have a circular course around the 

 attachment of the iris. They are sometimes called the " ring muscle " of Miiller, 

 and were formerly described as the ciliary ligament. The Ciliary muscle is admitted 

 to be the chief agent in accommodation i. e. in adjusting the eye to the vision of 

 near objects. Mr. Bowman believed that this was effected by its compressing the 

 vitreous body, and so causing the lens to advance ; but the view which now 

 prevails is that the contraction of the muscle, by drawing on the ciliary processes, 

 relaxes the suspensory ligament of the lens, thus allowing the anterior surface 

 of the lens to become more convex. The pupil is at the same time slightly 

 contracted. 1 



The Retina 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 

 choroid, the inner surface with the vitreous body. Behind it is continuous with 

 the optic nerve ; it gradually diminishes in thickness from behind forward, and 

 in front extends nearly as far forward as the Ciliary muscle, 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 Iriteus (macula 

 luted) of Sbmmerring, having a central depression at its summit, the fovea 

 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 

 Sbmmerring " at first given to it. It exists only in man, the quadrumana, and 

 some saurian reptiles. About one-tenth of an inch to the inner side of the yellow 

 spot is the point of entrance of the optic nerve ( porus opticus) ; here the nervous 

 substance is slightly raised so as to form an eminence (colliculus nervi optici); the 

 arteria centralis retinae pierces its centre. This is the only part of the surface of 

 the retina from which the power of vision is absent. 



Structure. The retina is an exceedingly complex structure, and, when exam- 

 ined microscopically by means of sections made perpendicularly to its surface, 



1 See explanation and diagram in Power's Illustrations of Some of the Principal Diseases of the Eye. 

 p. 590. 



