on Muscular Motion . 
*9 
are continued on to the edge of the cornea, and terminate, or 
are inserted, in its external lamina; their action will therefore 
extend to the edge of the cornea. 
4th, That in changing the focus of the eye from seeing with 
parallel rays to a near distance, there is a visible alteration 
produced in the figure of the cornea, rendering it more con' 
vex ; and when the eye is again adapted to parallel rays, the 
alteration by which the cornea is brought back to its former 
state is equally visible. 
Having supported these facts by the evidence of anatomical 
structure, and absolute demonstration, I shall consider them to 
be established ; and make some observations upon the muscular 
and elastic power by which so very curious an effect as the ad- 
justment of the eye is produced. 
The four straight muscles of the eye are attached to the bot- 
tom of the bony orbit near the foramen opticum ; they become 
broader as they pass forward, and when arrived at the anterior 
part of the eye-ball, are insensibly changed for tendons ; these 
adhere to the sclerotic coat, and terminate in the external 
lamina of the cornea, which appears to be a continuation of 
them. 
When we consider the situation of these muscles, it is evi- 
dent that their action will produce three very different effects 
upon the eye, according to circumstances. When they act se- 
parately, they will move the eye in different directions ; when 
together, with only a small quantity of contraction, they will 
steady the eye-ball ; and when this is increased they will com- 
press the lateral and posterior parts of the eye. This compres- 
sion of the eye will force the aqueous humour forwards against 
the centre of the cornea, while the circumference is steadied 
D2 
