Structure of the Eyes of Birds. 2 65 
from the eye, and this will be in proportion to the degree of 
relaxation. 
There seems to exist in nature an oeconomy of motion, to 
prevent fatigue, and exhaustion of the animal powers, by con- 
tinued voluntary muscular action. If two opposite actions of 
the same frequency occur in two muscles, the one being anta- 
gonist to the other, the action of one ceasing, the action of the 
other must take place previously to farther motion of the part ; 
for instance, on the biceps flexor of the arm acting, the arm 
will be bent, but on discontinuing its action the arm will re- 
main in the same state, unless it was straightened by the ac- 
tion of the biceps extensor, its antagonist : but where one 
action in a part is required to take place almost constantly, 
and the opposite action but seldom, to save the animal from 
fatigue, necessarily induced by muscular contraction, she gives 
an elastic ligament, which from its elasticity may be said to 
be in continual action, without exhausting the animal. Thus 
when the -opposite action which is of less frequent occurrence 
is required, it is performed by overcoming the resistance, or 
elasticity of this elastic ligament, which on the muscle giving 
over its action again, resumes its former state. The elastic 
cartilages of the ribs, performing in some degree the functions 
of a muscle, are of use in respiration ; likewise the elastic li- 
gaments which support the claws of all the feline genus, keep- 
ing them from friction against the ground. These claws, at 
the volition of the animal, by muscles appropriated for that 
purpose, are brought into action, or extended. From the 
abovementioned structure, the same thing appears to take 
place in the eyes of animals. When an animal is desirous of 
seeing minute objects, the recti muscles act, and thus, by 
