PHENOMENA Or MUSCULAR MOTION. 289 



strongly developed than those of the legs, in birds which are 

 much upon the wing ; the reverse taking place in the ostrich, 

 cassowary and penguin, which employ the wings only as akls to 

 the feet. 



Notwithstanding the great attention that has been paid to this 

 important operation of muscular fibres, the immediate cause is 

 yet unknown. 



Muscular motion takes place under the following different 

 circumstances : 



1st. When irritation or stimulus is applied directly to the 

 muscular fibre. 



2d. When irritation is applied to a nerve connected with the 

 muscles. 



3d. When it is induced by volition. 



There are several causes of muscular action which cannot be 

 arranged under either of these heads, although it is probable they 

 are not essentially different ; such as the motions of coughing 

 and sneezing, of yawning, &c. 



The immediate irritation of a muscle is effected by every 

 mechanical process, which punctures, divides, lacerates or 

 extends its fibres ; by acrid, and, perhaps, other chemical and 

 peculiar qualities of the substance applied to the muscles ; by 

 a sudden change of temperature; and by electricity and gal- 

 vanism. 



No satisfactory explanation has yet been made of the man- 

 ner in which muscular contraction is excited, either by the above- 

 mentioned agents, by irritation applied to a nerve, or by voli- 

 tion. 



When a muscular fibre begins to contract, there is often the 

 appearance of a slight tremor in it. It becomes hard and rigid : 

 its length diminishes, and its diameter increases. If a muscle 

 makes an effort to contract, when the parts to which its extre- 

 mities are attached are prevented from moving towards each 

 other, so that contraction cannot take place, the muscle will 

 become hard and rigid notwithstanding. 



This tremor of the fibres, is called fibrillary agitation, (agi- 

 tation fibrillaire) and is heard when a stethoscope is applied 

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