74 



APPARATUS OF MOTION. 



obvious motions, are due to the influence of light, heat, 

 moisture, cold, and similar external agents ; but all the mo- 

 tions peculiar to animals are produced by a cause residing 

 within themselves, namely, the contractility of muscular 

 fibres. 



151. The cause which excites contractility resides in the 

 nerves, although its nature is not precisely understood. 



We only know that each 

 muscular bundle receives 

 one or more nerves, whose 

 filaments pass at intervals 

 across the muscular fibres, 

 as seen in Fig. 25. It has 



also been shown, by experi- 

 ment, that when a nerve 

 entering a muscle is sev- 



Fig. 25. 



erecl, the muscle instantly loses its power of contracting 

 under the stimulus of the will, or, in other words, is par- 

 alyzed. 



152. The muscles may be classified, according as they 

 are more or less under the control of the will. The con- 

 tractions of some of them are entirely dependent on the will, 

 as in the muscles of the limbs used for locomotion. Others 

 are quite independent of it, like the contractions of the heart 

 and stomach. The muscles of respiration ordinarily act inde- 

 pendently of the will, but are partially subject to it : thus, 

 when we attempt to hold the breath, we arrest, for the mo- 

 ment, the action of the diaphragm. 



153. In the great majority of animals, motion is greatly 

 aided by the presence of solid parts, of a bony or horny 

 structure, which either serve as firm attachments to the 

 muscles, or, being arranged so as to act as levers, to in- 

 crease the precision and sometimes the force of movements. 

 The solid parts are usually so arranged as to form a sub- 



