STRUCTURE OF MUSCLES. 125 
two classes of exciting causes in ordinary circumstances. In 
some cases, it is true, we can acquire by habit a controut 
even over the involuntary muscles; and the muscles which 
are voluntary in one animal, may be involuntary in another. 
We may, however, add, as peculiar characters by which 
they may be distinguished, that the involuntary muscles 
are more durable in their action, more easily excited, and 
retain, even when separated from the body, their facility of 
irritability for a great length of time ; while the volunta- 
ry muscles become fatigued by continued action, and 
require intervals of rest to recruit their exhausted ener- 
gies. 
When the fibres of a muscle are excited to action by any 
irritating cause, a simultaneous movement is performed. 
They become shorter, more rigid, and in many cases ap- 
pear somewhat angular. In consequence of this contrac- 
tion, the two extremities of the muscle approach, bringing 
along with them the parts to which they are attached. In 
what manner the particles of the muscular fibre arrange 
themselves during contraction, anatomists have been unable 
to discover. That they approach each other, is obvious ; 
and that their cohesive power suddenly increases, appears 
evident, from the small force which prevents a muscle from 
contracting, compared with the prodigious resistance which 
it offers when contracted. Dr Wot.aston is of opinion, 
that each muscular effort, apparently single, consists in re- 
ality, of a great number of contractions, repeated at ex- 
tremely short intervals; so short, indeed, that the mterme- 
diate relaxation cannot be visible, unless prolonged be- 
yond the usual limits, by a state of partial or general debi- 
lity. We arrived at this conclusion, by attending to the 
sounds perceived in the ear upon the insertion of the tip of | 
the finger. It is probable, however, that these sounds are 
