CHAPTER VL 

 THE SKELETON. 



Exoskeleton and Endoskeleton. The skeleton ol an 

 animal includes all its hard protecting or supporting parts, 

 and is met with in two main forms. One is 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 although 

 the latter lie within the mouth, the study of development 

 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 con- 

 sists of a great many bones and cartilages or gristles, the 

 bones forming the mass of the hard framework of the Body, 

 while the cartilages finish it off at various parts. This frame- 

 work is what is commonly meant" by the skeleton; it pri- 

 marily 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 tissues, which not only, in the shape of 

 tough bands or ligaments, tie the bones and cartilages to- 

 gether, but also in various forms pervade the whole Body as 

 a sort of subsidiary skeleton running through all the soft 

 organs and forming networks of fibres around their other 

 constituents; they make, as it were, a microscopic skeleton 

 for the individual modified cells of which the Body is so 

 largely composed, and also form partitions between the mus- 

 cles, cases for such organs as the liver and kidneys, and 

 sheaths around the blood-vessels. The bony and cartilagin- 

 ous framework with its ligaments might be called the skele- 



63 



