450 VOLUNTARY AND INVOLUNTARY ACTIONS OF MUSCLES. 
more or less under the control of the will, they may all at 
times have an involuntary action. The heart and the muscular 
coat of the alimentary canal, with the muscles concerned in 
swallowing and in one or two other actions of a similar 
character, are the only muscles which the will cannot either 
set in action, or control when in action. There are several 
_ muscles whose usual movements are of a reflex and therefore 
involuntary character, and are yet capable of being, to a 
certain extent, controlled and governed by the will. Such 
are the movements of respiration; which will continue to 
take place after the brain has been removed, and which go on 
regularly during the profoundest sleep and the most complete 
withdrawal of the attention from them. In the Invertebrated 
animals these motions are probably not influenced by the 
will ; but in the air-breathing Vertebrata they are placed in a 
certain degree under the dominion of the will, in order that 
they may be made to contribute to the production of the vocal 
actions of speaking, singing, &c., which are restricted to these 
classes. "We can hold the breath for a certain time by a 
voluntary effort, or we can expel or draw it in more quickly 
than usual; but no voluntary effort can cause the breath to 
be held for more than a few moments; for the uneasiness 
which is then felt, and which is continually increasing, causes 
an involuntary action of the muscles, by which action it is 
relieved. 
590. But again, there are other muscles, whose ordinary 
actions are voluntary; but which are occasionally made to 
act independently of the will, or even against its direction. 
Such are those which are excited by the emotions, as in 
laughing, crying, sobbing, &c. "We may have the strongest 
desire to check these actions, owing to the unfitness of the 
time and place for their manifestation ; and yet we may be 
unable to do so. And lastly, muscles whose action is usually 
voluntary may be occasionally called into powerful contrac- 
tion, which the will cannot in the least degree control or 
prevent ; this is the case in cramps, convulsions, &c., of 
various kinds.—All these facts are readily accounted-for by 
the knowledge we now possess, as to the functions of the 
different parts of the nervous centres from which the muscles 
receive their stimulus to action (Chap. x.). 
591. The vigorous action of the muscular structure is de- 
