CHAPTER VI. 



THE SKELETON. 



Exoskeleton and Endoskeleton. The skeleton of an 

 animal includes all its hard protecting or supporting parts, 

 and is met with in two main forms in the animal kingdom. 

 First as an exoskeleton developed in connection with either 

 the superficial or deeper layer of the skin, and represented 

 by the shell of a clam, the scales of fishes, the horny plates 

 of a turtle, the bony plates of an armadillo, and the feathers 

 of birds. In man the exoskeleton is but slightly developed, 

 but it is represented by the hairs, nails, and teeth; for al- 

 though the latter lie within the mouth, the study of devel- 

 opment shows that they are developed from an offshoot of 

 the skin which grows in and lines the mouth long Before 

 birth. Hard parts formed from structures deeper than 

 the skin constitute the endoskeleton, which in man is highly 

 developed and consists of a great many bones and cartilages 

 or gristles, the bones forming the mass of the hard frame- 

 work of the Body, while the cartilages finish it off at vari- 

 ous parts. This framework is what is commonly meant by 

 the skeleton; it primarily supports all the softer parts and 

 is also arranged so as to surround cavities in which delicate 

 organs, as the brain, heart, or spinal cord, may lie with 

 safety. The gross skeleton thus formed is completed 

 and supplemented by another made of the connective tissue, 

 which not only, in the shape of tough bands or ligaments, 

 ties the bones and cartilages together, but also in various 

 forms pervades the whole Body as a sort of subsidiary 

 skeleton running through all the soft organs, forming net- 

 works of fibres around their other constituents; so that it 

 makes, as it were, a microscopic skeleton for the individual 

 modified cells of which the Body is so largely composed, 



