ANATOMY AND PHYSIOLOGY. 95 
the venom into the wound. All these, to mention no 
more, are marked by distinct transverse strie—the 
anatomical character of voluntary muscles throughout 
the animal kingdom. But the bulky muscles which 
move the wings in flight are of a different kind: the 
fibres of which they are made up are regular and 
have each their separate sarcolemma, but they display 
no distinct transverse strie. Some kind of trans- 
verse marking is to be observed, arising, apparently, 
from the coincidence of the divisions of the granules of 
which the different filaments are made up, and these 
lines are three or four times as closely set as the 
strie on the voluntary muscles. But these trans- 
verse markings are neither regular nor constant, and 
the difference between the two orders of fibres 
catches the eye at once. We find fibres of this 
order in the vertebrate uterus, or bladder, or bowels. 
Speaking generally, wherever a frequent or long- 
continued repetition of similar movements is re- 
quired, this function is made independent of the will, 
so that it may go on without interruption from 
weariness, or other causes which affect the will; and 
its performance is committed to organic or involun- 
tary muscles. Such are the muscles of flight in 
insects. 
Nothing is omitted in the adaptation of the muscles 
of flight in insects to their work. Their bulk at once 
bespeaks their strength. By their organic structure 
they are fitted for movements, which are unchanged 
except in the rapidity of the vibrations, and to labour, 
which is unremitting hour after hour. They are 
protected from the sense of wearmess which affects 
voluntary muscles, and they are supplied most freely 
