476 



THE FORMS OF TISSUES 



[CH. 



wall running between the other two; and where the partition t 

 meets the outer wall TT\ let the several tensions, or the tractions 

 exerted on a point at their meeting-place, be proportional to T, T 

 and t. Let a, ^, y be, as in the figure, the opposite angles. Then: 



(1) If T be equal to T' , and t be relatively insignificant, the 

 angles a, ^ will be of 90°. 



(2) If r = T\ but be a little greater than t, then t will exert 

 an appreciable traction, and a, ^ will be more than 90°, say for 

 instance, 100°. 



(3) lfT=T' = t, then a, ^, y will all equal 120°. 



Fig. I(i2. Part of a dragonfly's wing. 



The outer walls of the two cells on either side of the partition 

 will be straight, as well as continuous, in the first case, and more 

 or less curved in the other two. We have a vivid illustration (if a 

 somewhat crude one) of the first case in a section of honey : where 

 the waxen walls, which meet one another at 120°, meet the wooden 

 sides of the box at 90°. 



The wing of a dragon-fly shews a seemingly complicated system 

 of veins which the foregoing considerations help much to simplify. 

 The wing is traversed by a few strong "veins," or ribs, more or 

 less parallel to one another, between which finer veins make a 



