20 
Mr. Home's Lecture 
The bony rim, to which the muscles are attached, confines 
the effect of their pressure to the broadest part of the eye; 
and as their action throws forwards the cornea, the anterior 
edge of the bony rim yields, to adapt itself to that change ; 
the ciliary processes are long, to admit of the lens being moved 
forwards, and by their action bring it back to its place ; by 
these means the eyes of birds are adjusted to see very near ob- 
jects with more facility than the eyes of other animals. 
As the eyes of birds are likewise to be adjusted to see very 
distant objects, the marsupium is placed behind the crystal- 
line lens, to draw it backwards, and when it acts, part of the 
pressure from behind being removed, the cornea is rendered 
flatter ; and the anterior edge of the bony rim is adapted to 
it, in this state, by the contraction of the annular elastic liga- 
ment. 
It may be said, that to see with parallel rays no such great 
change is necessary ; it must, however, be considered that 
where vision is to be very distinct, a certain nicety of adjust- 
ment becomes necessary, and the action of the marsupium is 
probably intended for that purpose. 
In the bird (although not immediately connected with 
the present subject) there is one of the most beautiful illustra- 
tions of the combination of muscular and elastic substances. 
This is employed for the motion of the membrana nictitans, 
and as it shews that such a combination is adopted wherever 
it can be used with advantage, and is provided as a defence 
for the organ in which I am endeavouring to explain such a 
combination, I cannot avoid taking notice of it. The mem- 
brana nictitans is composed of an elastic membrane, which is 
connected by means of a tendon, with two muscles situated 
