CHAPTER V 

 THE SKELETON 



Exoskeleton and Endoskeleton. The skeleton of an animal in- 

 cludes 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 repre- 

 sented 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 struc- 

 tures 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 various 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 tissues, which not only, in the shape of tough bands or 

 ligaments, tie the bones and cartilages together, 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 fibers 

 around their other constituents; they make, as it were, a micro- 

 scopic skeleton for the individual modified cells of which the Body 

 is so largely composed, and also form partitions between the muscles, 

 cases for such organs as the liver and kidneys, and sheaths around 

 the blood-vessels. The bony and cartilaginous framework with its 

 ligaments might be called the skeleton of the organs of the Body, 

 and this finer supporting meshwork the skeleton of the tissues. 



The Bony Skeleton (Fig. 17). If the hard framework of the 



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