THE SKELETON 



259 



STRUCTURE OF BONE 



Shape and General Structure. The shape of bones is de- 

 signed to render them as strong and yet as light as possible. 

 Upon the long bones of the arms and legs comes the heaviest 

 strain, and consequently they are the ones most frequently 

 broken. Every bone is covered on the exterior with a thin 

 connective tissue membrane called 

 the periosteum. This is full of 

 blood vessels and by means of the 

 material thus brought to the bone, 

 new matter is added to it so that 

 this membrane plays a very impor- 

 tant part in the growth of the 

 skeleton. 



If a bone is cut open lengthwise, 

 it will be seen to be hollow through- 

 out most of its length, while the 

 bone itself, especially in the middle 

 of the shaft, is very hard and 

 tough. This arrangement in a 

 hollow cylinder is known to give the 

 greatest amount of strength possible 

 with the amount of material used. 

 The ends of the bone which are not 

 hollow are very much larger than 

 the shaft, in order to furnish suffi- 

 cient surface for the joint and also for the muscles, which 

 are attached to the bone near the end. The larger the surface 

 the more chance for the attachment of muscles and the 

 stronger their action. If a bone at its enlarged ends were of 

 the same dense structure as in the middle of the shaft, the 

 whole would be very heavy; to avoid this without materially 

 weakening it, the ends are spongy and porous. 



Marrow. The cavities in the middle of the long bones are 



FIG. 132. THE BONES OP 



THE FOOT 

 (Thompson) 



