26 THE HUMAN BODY. 



been possible to demonstrate this double movement of nutri- 

 tion. If we mingle madder with the food of an animal, the 

 bones soon become red, and they regain their original colour 

 when the colouring matter ceases to form part of the food. 

 Or, more conclusive still, if the madder be given for a while 

 and then omitted, and after a time be again given, the bones 

 show a white layer between two red ones, which proves that 

 they grow from the circumference toward the centre, by the 

 ossification of the deepest layers of the periosteum. 



The bones are divided according to their form into long, 

 flat, and short bones. The long bones, which are first and 

 most rapidly developed, are more dense in the middle or 

 body of the bone than at their extremities. This body is 

 formed principally of a compact bark or rind of ivory-like 

 tissue, and is pierced throughout its length by the medullary 

 canal; the extremities are composed of spongy tissue en- 

 veloped by a thin layer of ivory tissue. The long bones 

 combine to form the limbs and the thorax; designed to 

 serve as levers or columns, they are twisted on their axes or 

 bent so as to offer the greatest possible resistance in exert- 

 ing a force or supporting a weight. 



The flat bones form the walls of the cavities of the 

 cranium, of the chest, and of the pelvis; they are thinner in 

 the middle than at the edges, and are composed of two 

 leaves or tables of ivory tissue resting upon and confounded 

 with each other at some points, and separated at others by 

 a layer of spongy tissue. 



The short bones are very irregular in form and difficult to 

 describe; very spongy in texture and relatively light. They 

 develop later and more slowly than the others, and they are 

 placed in groups in parts of -the body where the bony frame 

 must possess a limited power of motion,- and yet be very 

 firm, as in the foot, in the hand, and in the spinal column. 



There are 198 bones in the human skeleton when it has 

 reached its perfect development. On the surface of the 

 bones, and especially at the extremities of the long ones, 

 there are prolongations of different forms, designed either to 

 join the bones together, or to serve for the attachment and 

 insertion of muscles or ligaments. These prolongations are 



