
508 BIRD’S-EYE VIEWS. 




















zontal 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 grow 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 at — 
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 - q 
ball to the optic nerve, 7, where it ends in a broad flat trans- 
verse tendon, not attached to anything, but perforated so a 
to form a sheath or loop; h is the pyramidalis musele, also 
so-called from its shape, tapering into a very long thread- 
like tendon, %, that first runs through the pulley-like sheath 
in the tendon of the quadratus, and then curves downwari — 
and backwards over the ball, to the margin of the latter. E a 
winds around, gets in front of the ball, and goes to be I- 
serted into the lower corner of the nictitating membrane. : 
If this slender tendon went straight along to the margin of 4 
the ball, and across the front, it would be right in the lise — 
of vision when the nictitating membrane, retiring to! o 
ner, pulled it after. If it went directly under the ball to get 
to the front, it would not have the right direction to gs 
the membrane straight across the eye. So it must T 4 
around the optic nerve. But now it would press Upe?» if 
interfere with the all-important functions of, the ee 
there were no provision for keeping it away from the pa a 
when the pyramidalis exerts its force of traction. Here 
quadrate muscle comes into beautiful action ; it always eh | 
tracts simultaneously with the pyramidal, and weg : 
tendon of the latter up out of the way of the nerve. 
is the ingenious, concerted action, of these tw? " 



