512 PHYSIOLOGY 



of this reciprocating mechanism must greatly increase the efficiency with 

 which the pupil works. 



The above experiment also shows that there is a connecting nerve 

 path between the pupilo- motor centre and the cortex of the brain. The 

 reaction of the pupil to light and the association which exists between pupil, 

 accommodation and convergence indicate that there are a number of other 

 important connections between the pupilo-motor and other centres. The 

 more important of these will be therefore traced. 



The light reflex in certain animals such as frog and eel is assisted by a 

 local sensibility of the iris to light, while in birds on the other hand it is 

 to a considerable extent under voluntary control ; but in man and in most 

 other higher vertebrates the control is involuntary and unconscious, the size of the 

 pupil being determined by the intensity of the light which is reaching the retina. Thus 

 in bright light the puj)il may be less than O6 mm. in diameter, while in the dark it may 

 be larger than 10 mm. ; such a change will cause the intensity of the light with the 

 pupil contracted to be nearly one two-hundredth part of the intensity when the pupil 

 is dilated. This indicates an extraordinary range of variation in the length of the 

 sphincter muscle fibres. The reaction of the pupil to light varies with the rate at which 

 change of intensity occurs. When the alteration is sudden the amount of contraction 

 was found by Haycraft to be equal to the logarithm of the intensity of the light. When 

 however the alteration is so gradual that the retina can become adapted to the change 

 as it proceeds, then little or no change in the size of the pupil occurs. The function 

 of the pupil appears rather to protect the retina from any sudden change of intensity, 

 than to control the actual intensity of the light. With regard to the reflex arc the 

 connection of the iris with the pupillo-motor centre has already been described. The 

 connections of the retina with this centre appear to traverse the following course. 

 Starting from the retina on one side the impulses travel up the optic nerve as far as the 

 chiasma, where they travel on without decussating, and end by anastomosing with nerve 

 cells in the anterior corpora quadrigemina. These nerve cells correspond with sen .ml 

 order neurons and proceed to the pupillo-motor centres of both sides. It is found by 

 experiment in monkeys that dividing the chiasma in the middle line does not stop either 

 the pupil reflex in the eye stimulated or in the other eye. This shows that the nerves 

 concerned with the pupil reflex go to the anterior corpora quadrigemina of the same 

 side, and that the consensual reflex is due to the fibres from each anterior corpus quad- 

 rigeminum supplying the pupillo-motor centres of both sides. The appreciation of light by 

 tin- retina is exceedingly rapid, whereas the response of the pupil to light action is very 

 delayed. This is due in part to the fact that the muscles of the iris are composed >f 

 involuntary, smooth fibres. There is however a pathological condition called Hippus, in 

 which the pupil alternately expands and contracts at a rate that would be impossible if 

 the attempt were made to produce this effect by alternately exposing the eye to 1 it'll t. 

 This proves that the muscle fibres can react more quickly and therefore that then- is 

 somewhere in the reflex arc a delay action mechanism. Th object of this mechanism 

 would appear to l>e to render the pupil stable and to prevent 'hunting.' When this 

 mechanism is diseased Hippus results. 



The accommodation reflox has been already considered. The close anatomical 

 association of the 3rd nerve con f res fur pupil sphincter, necotnmodat ion and com er^ence 

 by the internal recti is shown in Figure 247. When volitional impulses therefore 

 come down via the frontal lobes of the cerebral hemispheres, they are conveyed to this 

 group of centres, and the associated reflex results. 



ARGYLL - ROBERTSON PUPIL. The diagnosis of interference 

 wit.li t!io pupillo-motor reflex is of considerable practical import- 

 ance, because these paths appear to be particularly sensitive to 



