GOVERNMENT OF THE IRIS. 447 



necessary for near vision. It is through the oculo-motor nerve, which 

 supplies the sphincter of the pupil as well as the internal recti muscles, 

 that this consentaneous narrowing of the pupil is excited. But the 

 one movement is independent of the other, because the contraction of 

 the iris, and the accommodation movements, occur equally well, when 

 one eye only is used; and, by some persons, the eye can be accom- 

 modated without any change in the degree of convergence. Lastly, 

 certain cases have been recorded in which the movements of the iris 

 were, in some manner, perhaps indirectly, under the control of the 

 will. 



The movements of the ciliary muscle, like those of the iris, with 

 which they appear to be consentaneous, are usually involuntary ; they 

 are probably, however, sometimes independent of each other. The 

 nerves which regulate the action of the ciliary muscles are supposed 

 to be branches of the third pair, but this is uncertain. In some ex- 

 ceptional cases, the ciliary muscle has exhibited an apparent subjection 

 to the control of the will. 



The nervous centres concerned, are the anterior pair of the corpora 

 quadrigemina; the aiferent nerves are the optic nerves; and the effer- 

 ent nerves are the third cranial, and the sympathetic. The dilatation 

 of the pupil is regulated by nervous influence, conveyed from the spinal 

 cord, through the branches of the sympathetic nerve, which supply 

 the radiating fibres, division of that nerve in the neck being followed 

 by contraction of the pupil, and its irritation by dilatation. The con- 

 traction of the pupil is governed by the circular fibres, and is regulated 

 through the third cranial nerve alone. Irritation of the latter nerve 

 causes narrowing, whilst its division is followed by dilatation of the 

 pupil. The diameter of the pupil at any particular moment, however, 

 depends on certain combinations of action of its radial and circular 

 fibres. Its state of complete repose, or quiescent condition, as, for 

 example, when no light is present, or the optic nerves are diseased or 

 inactive, is that of moderate dilatation. After division of the optic 

 nerve, in an experiment, the same result is noticed; if then, the third 

 cranial, or oculo-motor nerve, which supplies the circular fibres, be 

 divided, the pupil does not widen; but if the sympathetic, which sup- 

 plies the radial fibres, be now cut, the pupil slightly contracts. From 

 this it would seem, that in moderate contraction, only the radial fibres 

 are really active. 



Of the different portions of the retina, the fovea centralis, which is 

 situated in the line of direct vision, is the part most sensitive to light, 

 and is the seat or area of distinct vision, both as regards form and 

 color. At the point of entrance of the optic nerve, the optic colliculis, 

 or optic eminence, which is not in the line of most perfect vision, the 

 retina is incapable of receiving distinct visual impressions. It has in- 

 deed been named the blind spot. If two small dots be made upon a 







piece of paper, at a short distance from each other, and the optic axis 

 of the right eye be directed vertically upon the left-hand dot, whilst 



