HOW ANIMALS MOVE. 155 



nerves go to them, yet they work in concert, waves of 

 motion passing over a surface covered with cilia, as over 

 a field of grain moved by the wind. 



But muscular tissue is the great motor agent, and exists 

 in all animals from the Coral to Man. 79 The power of 

 contractility, which in the Amoeba is diffused throughout 

 the body, is here confined to bundles of highly elastic 

 fibres, called muscles. When a muscle contracts, it tends 



FIG. 121 A Contracting Muscle. 



to bring its two ends together, thus shortening itself, at 

 the same time increasing in thickness. This shrinking 

 property is excited by external stimulants, such as elec- 

 tricity, acids, alkalies, sudden heat or cold, anu even a 

 sharp blow; but the ordinary cause of contraction is an 

 influence from the brain conveyed by a nerve. The prop- 

 erty, however, is independent of the nervous 

 system, for the muscle may be directly stim- 

 ulated. The amount of force with which a 

 muscle contracts depends on the number of 

 its fibres; and the amount of shortening, on 

 their length. 



As a rule, muscles are white in cold-blooded 

 animals, and red -in the warm-blooded. They 

 are white in all the Invertebrates, Fishes, 

 Batrachians, and Keptiles, except Salmon, 

 Sturgeon, and Shark ; and red in Birds and 

 Mammals, except in the breast of the com- Fl g G t ' r 



lllOn fowl, and the like. 80 larFibre,much 



, . . enlarged; n, 



It is also a rule, with some exceptions, that nucleus. 

 the voluntary muscles of Vertebrates, and all the muscles 



