464 PHILOSOPHICAL TRANSACTIONS. [aNNO 1795. 



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 evident that their ac- 

 tion will produce 3 very different effects on the eye, according to circumstances. 

 When they act separately, 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 compress the lateral and posterior parts 

 of the eye. This compression of the eye will force the aqueous humour for- 

 wards against the centre of the cornea, while the circumference is steadied by 

 the muscles, so that the radius of curvature of the cornea will be rendered 

 shorter, and its distance from the retina increased. That the eye-ball cannot be 

 made to recede in the orbit by any of these actions, is sufBciently proved by its 

 not having done so in any of tiie experiments. These muscles are uncommonly 

 large, and come much more forward than appears necessary for the purposes ge- 

 nerally assigned to them ; but when applied to so important an office as that 

 we have just stated, their size, and anterior insertion, are easily explained. 



It may be imagined that I have allotted to these muscles a greater variety of 

 uses than is compatible with the simplicity of the general laws of the animal 

 economy : but to prove this not to be the case, I shall only bring the biceps 

 flexor cubiti as an instance of a similar kind. That muscle is attached to the 

 scapula by both its heads, one of which passes through the joint of the shoulder, 

 they afterwards unite, and their common tendon is inserted into the radius ; 

 when the muscle contracts, the first effect will be to steady the joint of the 

 shoulder ; if the contraction be increased, it will rotate the radius, and if still 

 more increased, bend the fore-arm. 



There are many instances in animal bodies of elasticity being substituted for 

 muscular action, but this in the eye is by much the most beautiful of those appli- 

 cations. In the vascular system the arteries are composed of muscular fibres, 

 and an elastic substance ; in the natural easy state of the circulation, the re- 

 action in the larger vessels is principally the effect of elasticity ; but when in- 

 creased, it is the effect o( muscular contraction. The claws of the lion 

 are drawn up, and supported from the ground, by means of elastic liga- 

 ments ; but they are brought down for use, which is an action not so 

 often required, by muscles. In the adjustment of the eye it is the same ; the 

 state fitted for parallel rays is the effect of elasticity, but that for nearer dis- 

 tances, which is less frequently wanted, is the effect of muscular action. In 

 these different instances, the intention is uniformly to avoid the expence of 

 muscular action whenever the effect can be produced in any other way, as mus- 

 cular actions consume a considerable quantity of blood, which is the nourish- 

 ment of the body. That the adjusting the eye to near distances is the eflect 



