180 INTERNAL ANATOMY OF INSECTS. 
Chabrier seems to think that, in some cases, the back 
that intervenes between each pair of wings is the me- 
dium by which the muscles act upon it a . 
vi. Motions. Irritability is the universal distinction of 
the muscular fibre, — when put in action by the will or 
involuntarily, it causes it to contract or become shorter; 
and the intermediate agents of the will and other causes 
are the nerves, which, as galvanic experiments seem in 
some degree to prove, are the conductors of an invisible 
fluid or power which immediately causes that action. If 
a nerve is divided, the muscles to which it renders obey 
it no longer, evidently proving that the nerves cause 
muscular irritability 5 . How this contraction is imme- 
diately effected, — whether the fibre, as some suppose, 
undergoes any crispation, or becomes zigzag c , or whe- 
ther there is any sudden change in their chemical compo- 
sition that rapidly and strongly augments their cohesion, 
as Cuvier hints d , cannot be clearly ascertained, unless 
a Bauer could submit the living fibre to his glasses. All 
that we know certainly on the subject is, that muscles 
alternately contract and relax at the bidding of the will 
or involuntarily, and so occasion all the movements of 
animal bodies. 
II. Having considered the muscles of insects in gene- 
ral, I must next make a few observations, as far as my 
means of information will enable me, upon those that 
move their different parte and organs — at least the prin- 
cipal ones ; since to descend to minutiae would be an 
a Plate XXII. Fig. 1 1, 12. c. Chabrier Surle Voldes Ins. c. iii. 
/. xi. viii.y. 9. S. D. i, 1c. c. i. 440—. " Cuv. Anat. Comp. i. 94—. 
c AT. Diet. (Wist. Nat. xxii. 80. " Ubi supr. 101— . 
