68 A COURSE ON ZOOLOGY. 



CHAPTER VII. 

 Functions of Locomotion Organs of Speech. 



WE must now consider the skeleton as the solid frame- 

 work of the body, serving for the attachment of the 

 muscles, and furnishing points of resistance for their 

 contractions. 



The skeleton is a system of bones. Bones are com- 

 posed of a gelatinous matter, hardened by the deposit of 

 a calcareous mineral matter in its substance. In an 

 adult the gelatinous matter represents only a third of the 

 total weight of the bones, the remaining two-thirds con- 

 sisting of mineral matter ; but early in life the mineral 

 matter is present only in small proportion. Then many 

 parts of the bones have but a cartilaginous structure ; an 

 example is seen in the fontanels in the skull of a very 

 young child, and, on account of the close proximity of 

 the brain, shocks to the head are then very dangerous. 

 The gradual hardening of the bones by calcareous mat- 

 ter is called ossification, and ossification is only com- 

 pleted towards the time of the complete development of 

 the individual, that is, towards adult life. The bony sub- 

 stance does not present throughout its mass the same 

 characteristics. In some parts it is hard, compact, and 

 strong, resembling ivory ; in others it is spongy, being 

 perforated with holes throughout. Ordinarily the ex- 

 terior portions of the bones are made up of compact 

 tissue, while the interior portions and extremities are 

 composed of spongy tissue. 



The bones have been classified as of three kinds, ac- 



