THE BONES, OR OSSEOUS SYSTEM. 333 



dry up." When the bones are held together by their 

 own ligaments, it is called a natural skeleton; when 

 joined by wires, it is an artificial skeleton. 



Ossification. The formation of bone, which this term 

 signifies, consists in the deposition of the phosphate of 

 lime on the soft solids of animal bodies, by a natural 

 process. 



The bones undergo many changes before they reach 

 their entire hardness and strength. In the young animal 

 they are rio harder than cartilage, as may be observed in 

 the head of the infant. They gradually increase in con- 

 sistence and strength until about the twentieth year, 

 when the bones of our species are supposed to become 

 perfect for all the purposes for which they were design- 

 ed. At this period of life the bones are of full size, and 

 of sufficient hardness to sustain the action of all the mus- 

 cles. Besides the phosphate and carbonate of lime, they 

 contain large quantities of animal matter, and hence they 

 are not readily fractured. But in extreme age the earthy 

 matter predominates, and the bones become brittle by the 

 loss of their animal matter, and from this cause it is that 

 fractures are occasioned by slight accidents in old per- 

 sons. 



Eminences and Depressions of the Bones. Many of 

 the bones have eminences or projections, and some of 

 them furrows, or depressions, which are known to anat- 

 omists by certain names, and which, therefore, it is nec- 

 essary for the student to know. 



Eminences are called heads when they are convex, or 

 roundish, and smooth on the surface, as the head of the 

 femur, or thigh bone, and the humerus, or arm bone. 

 Condyle, which means a knuckle, is applied to the pro- 

 jections on the lower ends of the femur and humerus. 

 Tubercles, or tuberosities, are small prominences, as 

 those near the heads of the os humeri. Spine, or spi- 

 nous processes, literally thorn-like, are the projections 



At what age do the bones of man become perfect ? What is the composi- 

 tion of the bones ? Why are the bones of old persons easily broken ? What 

 are the condyles of the bones ? What are tubercles ? What are spines ? 



