m Muscular Motion. 
*9 
vent its contracting to the utmost ; while in the other, the lens 
being in its natural situation, could not allow of any unusual 
contraction. Some days after, the two eyes were examined ; 
in the perfect eye the marsupium measured ^ of an inch, and 
the different folds of it were semitransparent; in the imperfect 
eye the marsupium measured ^ of an inch* and the folds were 
much more opaque. Here, then, was a difference of ~ of an 
inch in the length of the two marsupia ; which could arise 
from no other cause than the one having contracted so much 
more than the other, which contraction we must consider as 
muscular. 
Haller denies the marsupium to be muscular, because 
there is no such appearance in its structure. My own opi- 
nions upon the structure of muscles have been already ex- 
plained to this learned Society ; and I have lately met with 
an observation in Lyonet's dissection of a caterpillar which 
tends to confirm them. He says, the muscles of the cater- 
pillar are, in their natural state, transparent as jelly, and have 
vessels passing through their substance in every direction, 
which afford to the eye of the observer in the microscope the 
most beautiful appearance of a congeries of vessels* 
The peculiarities in the bird's eye are such as tend to faci- 
litate both the lengthening of the axis of vision, and increas- 
ing the convexity of the cornea. 
* “ Les muscles des chenilles, dans leur etat naturel, ils sont mous, ils pretent ex- 
" tremement, ils ont la transparence d’une gelee, ils sont d’un gris bleuatre, et les 
“ branches argentees, ou vaisseaux aeriens, qu’on voit alors distinctement ramper par 
“ dessus, et penetrer dans toute leur substance, offrent a la loupe un spectacle qu’on 
te ne se lasse point d’admirer .” — Traite Anatomique de la Chenille , par Pierre 
Lyonet, chap. 6, page 92) 
