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 

 tli an 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- 



