412 I'HILOSOPHICAL TRANSACTIONS. [ANNO 1794. 



Such then I conceive to be the action and effects of the external muscles, and 

 which I apprehend will also apply in explaining many other phaenomena of vi- 

 siot} ; some of those it will not be improper at present briefly to notice. First, 

 may not the action of those muscles have more or less effect in producing the 

 changes of vision which take place in the different periods of life ? At the same 

 time the original conformation of the eye, the diminution of its humours, and 

 probably of the quantity of fat on which the eye is lodged, are also to be taken 

 into the account. But the external muscles becoming irregular and debilitated 

 by old age, in common with every other muscle of the body, are not only inca- 

 pable of compensating for these losses, but cannot even perform their wonted 

 action, and thus necessarily have considerable influence in impairing vision. 

 Again, does not the habit of long sight so remarkable in sailors and sportsmen, 

 who are much accustomed to view objects at a great distance, and that of short 

 sight, as of watchmakers, seal-cutters, &c. admit of an easy solution on this 

 principle? as we know of no part of the body so susceptible of an habitual action 

 as the muscular fibre. 



2dly, How are we to account for the weaker action of one eye in the case of 

 squinting ? That this is the fact has been well ascertained ; Dr. Reid * on this 

 subject observes, that he has examined above 20 persons that squinted, and found 

 in all of them a defect in the sight of one eye. Porterfield and Jurin have made 

 the same observation. The distorted position of the eye has I believe been ge- 

 nerally attributed to the external muscles ; but no satisfactory reason has ever 

 been given why the eye, directed towards an object, does not see it distinctly at 

 the same distance as with the other. The state of the iris here cannot explain 

 it, as it contracts and dilates in common with the other ; nor can we suppose 

 any muscles the lens might possess could have any effect, as they are not at all 

 connected with the nature of this disease. 



But the action of the external muscles, I apprehend, will afford us a satisfactory 

 explanation. When the eye is turned from its natural direction, for example, 

 towards the inner canthus, it is obvious that the adductor muscle is shortened, 

 and its antagonist, the abductor, lengthened ; consequently, as the abductor has 

 not the same power of contracting itself with the adductor, when the eye is di- 

 rected towards an object, their power of action being different and irregular, the 

 compression made on the eye and its humours must also be equally irregular, and 

 therefore insufficient to produce the regular changes in the refraction and shape 

 of the eye we have shown to be necessary in adapting it to the different distances 

 of objects. The effects produced by making a partial pressure on the eye with 

 the finger, or speculum oculi, before noticed, would also appear to favour this 

 explanation. 



• See his Inquiry into the Human Mind, page J2'2. 



