4 BIRDS-EYE VIEWS. 



zoiital commissure, this one sweeps horizontally, or a little 

 obliquely across the front of the ball, from the side next the 

 beak to the outer one. When not in action, it lies curled 

 up in the lower anterior corner of the orbit : when wanted 

 for use it is pulled over the eye by the action of two mus- 

 cles that OTow on the back of the ball. The mechanism of 

 its movements — the most perfect and ingenious that could 

 be imagined — may be clearly understood with the help of 

 the figure on the preceding page, which represents the back 

 of the right eyeball, with all its muscles. Two of these act 

 upon the nictitating membrane alone ; g is the quadratus 

 muscle, so called from its somewhat squarish shape, arising 

 at the upper margin of the ball, and extending down on the 

 ball to the optic nerve, ^, where it ends in a broad flat trans- 

 verse tendon, not attached to anything, but perforated so as 

 to form a sheath or loop ; h is the pjramidalis muscle, also 

 so-called from its shape, tapering into a very long thread- 

 like tendon, A;, that first runs through the pulley-like sheath 

 in the tendon of the quadratus, and then curves downwards 

 and backwards over the ball, to the margin of the latter. It 

 winds around, gets in front of the ball, and goes to be in- 

 serted into the lower corner of the nictitating membrane. 

 If this slender tendon went straight along to the margin of 

 the ball, and across the front, it would be right in the line 

 of vision when the nictitating membrane, retiring to its cor- 

 ner, pulled it after. If it went directly under the ball to get 

 to the front, it would not have the right direction to draw 

 the membrane straight across the eye. So it must wind 

 around the optic nerve. But now it would press upon, and 

 interfere with the all-important functions of, the nerve, if 

 there were no provision for keeping it away from the nerve 

 when the pyramidalis exerts its force of traction. Here the 

 quadrate muscle comes into beautiful action ; it always con- 

 tracts simultaneously with the pyramidal, and carries the 

 tendon of the latter up out of the way of the nerve. Such 

 is the ingenious, concerted action, of these two muscles, 



