Mr. Home's Lecture 
action, which they cannot keep up, being unable to remain long 
in the same state ; nor can they, after having been used for any 
time, return to this adjustment with the same exactness. 
The change that takes place in the eye at an advairced pe- 
riod of life, by which it loses its adjustment to very near, and 
very distant objects, does not arise from any defect in the 
muscles, as might at first be imagined, since that would not 
account for the eye being unable to see with parallel rays ; 
nor is there any obvious reason why these muscles should lose 
their powers, while others, which are not apparently so strong, 
if we may judge by their effects, retain their full action long 
after the eye has undergone this change. 
This defect in the eye I am led to believe is brought on by 
the cornea losing its elasticity as we advance in life, neither 
contracting nor being elongated to its usual extent, but re- 
maining in a middle state. That elastic substances in the body 
do undergo such a change may be well illustrated in the vas- 
cular system. The aorta is composed almost entirely of elastic 
substance, and there is probably no part of the body, at an ad- 
vanced age, which is so often found to have lost its natural 
action ; it appears to undergo a change from age alone, be- 
coming inelastic, and then taking on diseases of different kinds, 
as being ossified, or becoming aneurismal ; but in neither of 
these diseases is it found to be contracted, although often the 
reverse, and when disease has not supervened, the artery more 
commonly remains in the middle state. 
The cornea having similar properties must be liable to a 
similar change, but its action being less constant, and the 
power which it is to resist being weaker, the change will be 
probably more gradual and less in degree, but sufficient to 
