NUTRITION OF BONES 243 



lating blood in the bones. In general, bone is formed 

 by the bone cells located in the lacunte, and the cartilage 

 and lime out of which its main structure is formed are 

 to be understood as secretions produced by these cells 

 out of the food supplied to the cells by the blood. This 

 blood is brought into the bones through the periosteum 

 and the Haversian canals, and the canaliculi are simply 

 channels for permitting the distribution of the capillaries 

 and their contents to the bone cells. 



In the child also, many bones are separate, which in the 

 adult fuse together. In the skull, for example, the parie- 

 tals and frontals do not meet at a point in the top of the 

 skull for some time. This spot is called the fontanelle 

 and often is not completely closed until the child has 

 attained an age of two years. Again, the bones of the 

 sacrum are five in number and separate in the child; 

 likewise the four bones of the coccyx, and the breast bones. 

 All of these bones fuse by gi-owth, and such fusion is called 

 ankylosis. It is this tendency of bones to fuse so readily 

 in youth that makes a broken bone knit so much more 

 quickly in a child than in an adult; while it. old age the 

 fusing power is almost entirely lost. 



