814 THE SENSE OF SIGHT 



this time, the afferent paths having to do with the accommodation 

 reflex, are still free from these degenerative alterations. 



The Accommodation Reflex. — If a person is asked to accommodate 

 alternately for near and far objects, it will be noted that the size of 

 his pupil is decreased on near vision and increased on far vision. In 

 the former instance, the number. of rays entering the eye is diminished, 

 but not at all sufficiently to impair our power of being able to make 

 out the finer details of the object. This reduction in the size of the 

 beam of light is, of course, entirely in keeping with perfect refraction, 

 because the amount of light projected into the eye from any given ob- 

 ject, increases inversely as the square of its distance. This implies 

 that the phenomenon, constituting the accommodation reflex, is an 

 associated action and is closely interlinked with the muscular efforts 

 necessary for accommodation. These efforts consist in a convergence 

 of the eyeballs effected by the contraction of the two internal recti 

 muscles, and a contraction of the cihary muscles, rendering the lens 

 more convex. The afferent impulses which give rise to these reactions, 

 are, of course, chiefly intracerebral in their origin, and do not involve 

 the optic tract. Consequently, it appears that the constriction of the 

 pupil on near vision is due to the fact that those motor discharges 

 from the midbrain which evoke the contraction of the internal recti 

 and ciliary muscles, overflow and simultaneously activate the neigh- 

 boring center for the sphincter fibers of the iris. 



In sleep the pupils are constricted in spite of the fact that the eyes 

 are not stimulated by light. This fact may seem to be opposed to 

 the view just expressed, unless it is remembered that the axes of the 

 eyeballs are at this time turned inward and upward. Obviously, 

 therefore, the initial constriction of the pupil during sleep is an associ- 

 ated movement, akin to that arising on near vision; in other words, the 

 motor impulses which are required to deviate the eyeballs in the afore- 

 said direction, also implicate the sphincter muscle of the iris. 



The constriction of the pupil during the initial stage of anesthesia 

 by ether or chloroform may be explained in a very similar way, because 

 these agents give rise at first to a general excitation of the central ner- 

 vous system. As soon as this primary effect has weakened, the pupil 

 retains an intermediate size, but dilates immediately if the narcosis 

 is deepened or is carried beyond its physiological limit. This danger 

 point of narcosis may also be determined in other ways, for example, 

 by noting the intensity of the reflexes and, especially of those which are 

 usually preserved during sleep and moderate narcosis. The one most 

 commonly employed for this purpose is the corneal, consisting in a 

 closure of the eyelids upon mechanical stimulation of the cornea. 

 Among the agents which constrict the pupil, may be mentioned 

 opium, and its alkaloid morphin, as well as the alkaloids eserin or 

 physostigmin and pilocarpin. Among the dilators of the pupil 

 should be cited the alkaloids of belladonna, namely, atropin and homa- 

 tropin. A dilatation of the pupil commonly results in consequence 



