664 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



Q. 10 — 



z 

 o 



o 

 o 



o 



SPIKE 



LIMIT OF 

 POS.FUS.CON. 



50 100 



CONVERGENCE (CENTRADS) 



FIG. -i I . Relation between accommodation, accommodati\e 

 convergence and positive and negative fusional convergence in 

 a normal 20-year-o]d subject. 



dation is shown in figure 2 1 . At each level of accommo- 

 dation the phoria position of the two eyes represents 

 the position of rest that the eyes assume when one of 

 them is covered. At the same level of accommodation 

 the two eyes can be made to converge toward or 

 diverge away from this position of rest in response to 

 stimuli upon which the two eyes can fu.se, and the 

 extent to which this can be done without changing 

 accommodation gives us the limits of fusional con- 

 vergence. The phoria line shows the relation between 

 accommodation and accommodative convergence 

 when the eyes are freed from fusion stimuli. The re- 

 ciprocal of the slope of this line is called the ACA ratio. 

 The 'spike' at the maximum level of accommodation 

 which indicates that an increased amount of con- 

 vergence can develop at that level was regarded by 

 van der Hoeve & Flieringa (87) as evidence that the 

 ciliary mu.scle can keep on responding after the lens 

 reaches its limit to respond, van Hoven (unpublished 

 observations) has shown in six subjects that this is not 

 the case. He paralyzed one eye with homatropine and 

 when the eye partially recovered, he measured the 

 phoria at various levels of accommodation for each of 

 the two eyes and he also measured the amplitude of 

 accommodation of each eye by measuring the ac- 

 commodative response at the maximum level of con- 

 vergence. He showed that the amplitude is strictly 

 proportional to the ACA ratio. This means that the 

 lens is responding to changes in contraction of the 

 ciliary muscle at the maximum level of accommoda- 

 tion. Hence it is the muscle and not the lens which 

 determines the maximum le\'el of accommodation. 

 Fincham (30) independently performed the same 

 kind of experiment except that he has also used 

 physostigmine. This gives a hypermaximal spike 

 which proves that the limit lies with the muscle or its 

 innervation but not with the lens. 



One can explain the "spike" by assuming that the 

 ciliary muscle reaches the limit of its capacity to con- 

 tract before the cortical center controlling the triad 

 responses reaches the limit of its capacity to initiate 

 impulses. 



The effect of age upon the ACA ratio is important 

 (8, 18, 30, 40). If the ACA were proportional to the 

 amplitude it ought to change as the amplitude 

 changes with age. The studies made so far have not 

 confirmed this relation. This raises the question 

 whether the connection between accommodation and 

 convergence is a matter of habit instead of dependence 

 on some fixed anatomical arrangement. 



Morgan & Olmsted (67) have shown that stimula- 

 tion of the sympathetic supply to the eye produces a 

 relaxation of accommodation to the extent of about 

 0.75 D below what is commonly regarded to be the 

 zero level. Morgan (66) believed this eflfect to be 

 mediated by a change in blood volume of the ciliary 

 body, but Melton et al. (65) have demonstrated in a 

 ijloodless eye that the eflTect is still obtained. The 

 mechanism involved has yet to be identified. 



Night and Skv Mvnjna 



In total darkness and in the absence of a stimulus 

 to accommodation the refractive state of the eye comes 

 to rest at a higher level than occurs when the eyes are 

 looking at a test chart at 20 ft. (47; 48; 56; 57, p. 485; 

 71 ; 72; 84). The effect of low levels of illumination can 

 partly be explained (84, 89) by the aberrations of the 

 eye, but these explanations do not apply in sky 

 myopia (92). In sky myopia as well as in night myopia 

 an increase in accommodation of about i D above its 

 zero level is found to exist. Moreover, VVestheimer 

 (gi) has shown that under these conditions the ac- 

 commodation is not fixed but exhibits slow oscilla- 

 tions up to a diopter in amplitude. These fluctuations 

 have also been reported by Campbell (14) and by 

 Chin & Horn (16). 



VISU.\L FIELD 



The visual field of a gi\en eye is a conical space 

 with its apex at the center of the entrance-pupil which 

 contains the chief rays for all parts of the retina that 

 respond to light. In the ordinary use of the eyes a part 

 of the field of view is cut off h\ the nose, eyebrow and 

 cheek. 



The direction of a point in the field of \ icw may be 

 specified in terms of its radial direction and eccen- 



