132 MECHANISM OF THE MUSCLES. 



Fig. 24. 



e, the trochlear muscle, which arises at the bottom of the socket 

 of the eye, and passes upward and forward like the rope in 

 the figure 23, till it reaches the pulley at d, on the inner 

 edge of the bony rim around the front of the eye, where it is 

 turned backwards, and inserted into the top of the ball. 



T. There is quite as curious an example in the 

 wing of the bird. 



A. It is necessary the weight of the bird should 

 hang below the wings, so as to balance the body 

 in the air, and prevent it from turning over ; for the 

 weight of the body under the wings is the same as 

 .the ballast of a vessel under the sails. This requires 

 that the muscles which constitute the principal part 

 of the weight, should be disposed as much as possi- 

 ble upon the breast, and this principle we find to be 

 observed. Every one who has seen a fowl upon the 

 table, knows that upon the back, above the wing, 

 there is only a mere skin. But the question arises, 

 if the muscles, which are wanted to raise the wings, 

 ere situated beneath them, how are the wings to be 

 elevated in the act of flying ? As the muscle can 

 only contract downwards, how is this to produce a 



