STRUCTURE OF THE HUMAN BODY. 43 



vessels]! without any appearance of fibres or plates. The 

 different densities of bones (depends on the different mechani- 

 cal arrangements of the parts composing them^ 



11. The bones not only constitute the frame work of the 

 human fabric, but they also protect the vital organs, as the 

 heart and lungs, the brain and spinal marrow, and also con- 

 stitute a series of levers, by means of which, through the 

 agency of muscles, locomotion is performed. The bones in 

 man also furnish cavities for the secure lodgment of the deli- 

 cate organs of the senses, as the orbits of the eye, ear, mouth, 

 and nostrils. In man, and the higher order of animals, the 

 bones are for the most part in the interior of the body, and 

 when near the surface, as in the skull, they are covered by 

 muscles or membranes; bufin the Crustacea, insects, &c., the 

 bones compose an external case within which all the soft 

 parts are containe(Jf 



12/If we divide any of the long bones longitudinally, we find 

 two kinds of structure, the hard or compact, and the alveolar 

 or spongy/ Indeed there is no bone that does not exhibit to 

 some extent both of these structures ; the compact forming 

 its external, and the spongy its internal part. These two 

 formations are clearly seen in the bones which compose the 

 skull, (as in the following cut,) with the spongy or cancel- 



a. External plate. 5. c. Internal plates. 



lated structure between them. This will serve to illustrate 

 the structure of all the flat bo/ies. 



i 



13. The above cut represents a section of the thigh bone. 



