50 



APPARATUS OF MOTION. 





closing of the leaves and blossoms of plants, which are their 

 most 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 determines contractility resides in 

 the nerves, although its action is not precisely known. 



We only know that 

 each muscular bun- 

 dle receives one or 

 more nerves, whose 

 filaments pass across 



in the figure. It has 

 also been shown, by 



Fig. 25. experiment, that when 



a nerve going from the brain to a muscle is severed, the 

 muscle instantly loses its power of contracting, or, in other 

 words, is paralyzed. 



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 the muscles of the limbs which are used for locomo- 

 tion. Others are quite independent of it, like the con- 

 tractions of the heart and stomach. The muscles of res- 

 piration act independently of the will, but are partially sub- 

 ject to it ; thus, when we attempt to hold the breath, we 

 arrest, for the moment, 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, they 



