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. 
1. Muscle.—Muscular tissue is the great motor agent, and 
exists in all animals from the Coral to Man. 7 " 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, and 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 Reptiles, except Salmon, 
Sturgeon, and Shark; and red in Birds and 
Mammals, except in the breast of the com- Fl B ® , rip 1 e 2 |* 
inon fowl, and the like. 80 iarFibre,much 
. . enlarged; w, 
It is also a rule, with some exceptions, that nucleus, 
the voluntary muscles of Vertebrates, and all the muscles 
