TYPES OF EFFECTORS 125 



control is known technically as the adjustor mechanism, 

 which in all the higher animals consists of the nervous 

 system. 



(c.) Effectors. — Finally we have the apparatus which 

 makes the motions, to which is given the general 

 name of effectors. The advantage of the word " effector " 

 rather than the word " muscles " is that we have, even 

 in some of the higher animals, movements carried on by 

 means of cilia, and a broad term like effectors covers such 

 movements while the word muscle would fail to do so. 



Types of Effectors. — From the preceding paragraph 

 it is clear that cilia must be included in a complete listing 

 of the kinds of effectors found in the higher animals. 

 For our present purpose, however, it is desired to concen- 

 trate the attention on the more highly specialized and 

 more familiar kinds of effectors, the muscles. 



In the higher animals there are three distinct kinds of 

 muscle tissue which will be mentioned briefly here and 

 described in more detail further on. 



Skeletal Muscles. — Under this designation are in- 

 cluded muscles which move the bones of the skeleton, 

 hence the name. These muscles make up the great bulk 

 of our muscle substance. They are quick-acting and 

 powerful, and are responsible for our main bodily 

 movements. 



Smooth Muscles. — The second type of muscle, 

 known as smooth muscle, carries on most of the internal 

 movements of the body. This kind of muscle is much 

 more sluggish than skeletal muscle, and is also compara- 

 tively feeble. In most of the lower animals this is the 

 only type of muscle that is present. In the higher ani- 

 mals it is found in the walls of the alimentary tract, in 

 the walls of the blood vessels, and in a few other places 

 which need not be listed here. 



Heart Muscle. — In all of the higher animals the 

 heart muscle differs from any other kind of muscle, and 

 must therefore be given a separate classification. It is 



