XV. THE SKELETON. 



In the preceding pages we have considered at some 

 length the means by which nutriment and oxygen are 

 distributed to the various tissues of the body. It will 

 be well for us to consider now two of these most important 

 tissues, the bones and the muscles, since to their action 

 we owe aU our voluntary and involuntary movements. 



The body framework. — If we examine certain of the 

 lower animals, such as the jellyfishes and worms, we find 

 that, while they have a body form, that form is subject 

 to distinct changes when subjected to pressm-e. In other 

 words, they are compressible to a great degree. This is 

 owing to the absence of any hard matter in their bodies, 

 that is, they have no skeleton or framework. Other ani- 

 mals, such as the insects, lobsters, and starfishes have a 

 framework, but it is on the outside of the body. While 

 such a skeleton is preferable to none at all, it is an incon- 

 venient arrangement, since in most of these forms it does 

 not permit any growth on the part of the animal, without 

 splitting of the hard layers, and thus growth in these 

 animals is attended with periods when the soft parts of 

 the body are exposed. Skeletons such as these are called 

 exoskeletons. In the higher animals the problem of secur- 

 ing rigidity and protection of the soft parts, and at the 

 same time allowing growth, has been solved by the devel- 

 opment of an inner framework or endoskeleton of bone. 



By the aid of an apparatus known as the X-ray machine 

 (see Plate II) it is possible to take a picture of parts of 



221 



