662 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY' I 



However, in some subjects the same type of effect 

 occurs in lesser degree with submaximal amounts of 

 accommodation. 



Duane (21) showed that in a youns; man it takes 

 the same time for homatropine to begin to take effect 

 as in an older man and argued from this that the older 

 man has the same excess of ciliary capacity above the 

 capacity of the lens to respond as the young man. 



It is necessary to look in some other direction to 

 ascertain what fraction of the total ciliary contraction 

 a\ailable is required to produce a diopter of accom- 

 modation at different ages. The answer is to be found 

 in the study of the relation of accommodation to con- 

 vergence as described in the next section. 



It remains to be determined whether the lens layers 

 become nonpliai)le one by one from the center out 

 with the cortical layers remaining relatively unaffected 

 or whether all la\ers get progressively less pliable 

 with age but with the hardening process more de- 

 veloped at the center than at the cortex. The role 

 played by the tensile strength of the indi\-idual fibers 

 must also be considered. 



The slit lamp technique of observing or photo- 

 graphing the internal structure of the lens, if system- 

 atically used at different age levels, might throw some 

 light on this problem. It would also be important to 

 study at different ages the time characteristics of the 

 response of the lens to changes in tension of the 

 zonule (7). 



Ciliary Muscle Potential 



Schubert (76) has recently developed a method for 

 detecting and recording a potential which appears to 

 be generated by contraction of the ciliary muscle. 

 One electrode is applied to the .sclera o\er the ciliary 

 bodv and the other to some indifferent part of the 

 bod\-. Alpern (unpublished observation.s) has de- 

 scribed the relation of size of this potential to the 

 amount of accommodation in play. This new tech- 

 nicjue provides us with an opportunity to study the 

 lag of the response of the lens behind the changes in the 

 ciliary mu.scle. 



Innervation Controlling Accommodation 



Although the ciliary muscle is classified histologi- 

 cally as a smooth muscle and the branch of the third 

 nerve supplying it is identified as a part of the para- 

 sympathetic system, the ciliary muscle behaves in 

 many ways like a skeletal muscle. Marg et al. (64) have 

 demonstrated that in the cat the amount of accom- 



modation elicited in response to a square wave gal- 

 \anic current applied to the ciliary ganglion is a func- 

 tion both of the strength of the current and the fre- 

 quency of the stimuli. A submaximal response of any 

 degree may be elicited. 



Allen (2) has developed a method which can be 

 used in the cat or dog for comparing the lag of the 

 lens and the lag of the muscle. A faradic stimulus is 

 applied to the region of the ciliary muscle. A needle 

 pushed through the sclera into the choroid gives a 

 record of the muscle response, and motion picture 

 photography is used to measure the change in size of 

 the third Purkinje image. 



When the eyes accommodate, they also converge 

 and the pupils constrict. This is known as the triad 

 response. The convergence part of the triad response 

 is called accommodative convergence because it is 

 associated with accommodation. It has to be differen- 

 tiated from fusional convergence which is a different 

 kind of response. 



Allen (i) devised an arrangement for suddenly 

 switching from a stimulus at one distance to a stimulus 

 at a different distance. The stimuli were presented to 

 one eye only and the ensuing accommodative and 

 convergence responses were tracked with recording 

 devices. The accommodative response lags a little 

 behind the convergence response (see fig. 20) and 

 this may be due to the lag of the lens behind the re- 

 sponse of the ciliary muscle. The results confirm the 

 notion that the two types of responses are initiated 

 through a common center. 



The triad response is probably always brought into 

 plav bv voluntary effort and it behaves like a postural 

 adjustment of the arm which may be raised to a given 

 level and held in that posture while the subject pays 

 primarv attention to .some other aspect of his behavior. 

 Under normal conditions of use of the eyes, the only 



'^.46 



E 

 E 

 ~.53 



UJ 

 M 



u 

 ".73 



CONVERGENCE 



-^ 3rd PURKINJE IMAGE SIZES 



1 I I 1 1 L 



O 



.5 1.0 



TIME (SECONDS) 



IS 



FIG. 20. Accommodative and accommodative convergence 

 responses to a change in the stimulus to accommodation. The 

 points represent sizes of the third Purkinje image measured in 

 successive frames of a motion picture record occurring at a 

 rate of 64 per sec. [From Allen (i).] 



