ACCOMMODATION 529 



chiasm (man, monkey, predatory animals and some rodents) contraction of 

 both pupils results i. e., the reflex excitation passes over from One optic nerve 

 to both oculo motor nerves when light enters but one eye. 



Stimulation of other afferent nerves, and powerful respiratory movements, 

 as in dyspnoaa, etc., are as a rule accompanied by dilatation of the pupil, 

 although reflex constriction has also been observed with such stimulation. 

 Likewise stimulation of the most widely different parts of the brain, cerebral 

 cortex (motor zone and temporal convolutions), corpora striata, optic thalami, 

 anterior and posterior corpora quadrigemina, produces dilatation. Dilatation 

 of the pupil on stimulation of afferent nerves as well as on stimulation of 

 these various parts of the brain in many cases persists after bilateral section 

 of both the cervical sympathetic and the trigeminal nerves. It must be re- 

 garded therefore as, in part at least, the result of inhibition of the constrictor 

 center. 



The tonus of the constrictor nerves is mainly of reflex origin, for after 

 section of the optic nerve section of the oculo motor no longer produces dila- 

 tation of the pupil (Knoll). But stimulation of the optic nerve cannot be 

 the only cause of the sphincter tonus, for the pupil is strongly constricted 

 in sleep. 



The center for the constrictor nerves of the pupil is to be sought in the 

 nucleus of the oculo motor nerve. According to Hensen and Volckers, in the 

 dog it lies in the floor of the third ventricle close to the aqueduct of Sylvius and 

 a little posterior to the center for accommodation (Fig. 230). 



The center for the dilatation of the pupil was located by Budge in the 

 cervical cord (centrum cilio spiiiale) ; other authors have been led by their inves- 

 tigations to conclude that the center is located in the brain and that fibers pass 

 thence down the cord to the roots of exit. Since, however, after section of the 

 cord high in the neck, the pupil is dilated on stimulation of the sciatic nerve, 

 but is always constricted by section of the cervical cord alone, we may perhaps 

 safely infer that there is a center in the cord, the normal tonic influence of 

 which is to keep the pupil dilated. 



6. ACCOMMODATION 



A. RANGE OF ACCOMMODATION 



A point of light starting from the far point of the eye and brought gradu- 

 ally nearer to it, can be kept constantly focused on the retina. But this is 

 possible only up to a certain limit, there being for every eye a near point 

 whence it receives the most divergent rays which can be focused on the 

 retina. In order that these most divergent rays may be focused on the 

 retina the refraction of the eye must be increased in some way. The change 

 in the eye by which this is accomplished is called accommodation. Know- 

 ing the far point and the near point of the eye, it is very easy to calculate 

 how much the refraction is increased by accommodation. 



The focal distance of the lens which will give to the rays proceeding from 

 the near point the direction they would have if they came from the far point 

 furnishes the measure of the range of accommodation. 



