88 CONSCIOUS NERVOUS OPERATIONS 



32. Provision for Movement in Different Classes of Ani- 

 mals. A distinguishing feature of the animal kingdom 

 is the power of voluntary motion, and the ability at some 

 period of life to move about from place to place. For this 

 purpose the different classes and orders of animals are 

 provided with a great variety of mechanical devices. 

 Certain aquatic creatures propel themselves by means of 

 pulsations in the whole body. Snakes and worms swim 

 by the undulations of the body. The squid fills a certain 

 cavity within itself with water and then suddenly expels 

 it, and the force of the ejection moves the body in one 

 direction or another, according to the direction of the 

 current of water ejected. The jellyfish propels itself 

 by drawing in and expelling water from its bell-shaped 

 body. Animalcules move themselves by the rapid vibra- 

 tions of innumerable hairlike projections. But all the 

 higher forms of animals move by means of muscles and 

 ligaments attached to an internal solid framework, or 

 skeleton. 



33. The Vertebrate Skeleton. A careful study of any 

 vertebrate skeleton discloses marvelous adaptations for 

 accomplishing the two main objects of protection and 

 motion. Protection to the vital parts might have been 

 secured by means of a rigid, unyielding case, but in order 

 to allow motion in all parts of the body also, the skele- 

 ton is composed of many separate pieces united together 

 by elastic tissues. The whole number of bones in the 

 adult man is about 206, while in the child the number is 

 yet larger, because, as a child grows older, certain bones 

 which are at first distinct (and remain so in the lower ani- 

 mals) grow together to form one. For convenience in 

 study, the bony framework is divided into two parts called 

 the axial skeleton and the appendicular skeleton. 



