364 



COMPARATIVE ZOOLOGY 



:D y tiscus. 



The cause of ciliary motion is unknown. One-sided 

 contraction is their property, as straight contraction 

 is distinctive of the muscle fiber. No structure can, 

 however, be seen in them with the micro- 

 scope. No 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. 



i. Muscle. Muscular tissue is the great 

 FIG i -waves motor agent, and exists in all animals from 

 of Contraction the coral to man. 133 The power of con- 

 tractility, which in the amoeba is diffused 

 throughout the body, is here confined to 

 bundles of highly elastic fibers, called muscles. When a 

 muscle contracts, it tends to bring its two ends together, 

 thus shortening itself, at the same time increasing in 

 thickness. This shrinking property is excited by exter- 

 nal stimulants, such as electricity, 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 property, however, is inde- 

 pendent of the nervous system, for the 

 muscle may be directly stimulated. The 

 amount of force with which a muscle 

 contracts depends on the number of its 

 fibers; and the amount of shortening, on 

 their length. 



As a rule, muscles are white in cold- 

 blooded animals, and red in the warm- FIG. 

 blooded. They are white in all the 

 invertebrates, fishes, batrachians, and rep- 

 tiles, except salmon, sturgeon, and shark ; 

 and red in birds and mammals, except in the breast 

 of the common fowl, and the like. 134 



319. Un- 

 striped Muscu- 

 lar Fiber, much 

 enlarged; , nu- 

 cleus. 



