THE J1ONE8 AND JOINTS. 29 



store of nourishment which helps to sustain the body 

 when it is deprived of food. It has been proved by 

 experiment, that a hollow shaft is stronger than a 

 solid shaft of the same material and the same weight and 

 length. 



2. The bony substance of the shaft is hard and com- 

 pact. The large ends, on the contrary, though they are 

 not hollow, are filled with cells, and present a honey- 

 combed appearance. It is desirable that the shaft should 

 be slender and strong. Its substance is, therefore, very 

 compact. It is desirable that the ends should be large, 

 to give a broad surface for the joints. Their substance is, 

 therefore, open. If it were compact, it would increase the 

 weight unnecessarily. 



2, If we saw a flat bone in two, as one of the bones of 

 the skull, or a short bone as one of the bones of the wrist, 

 we shall find, that, 



while its shell is of 

 compact tissue, its in- 

 side is of the same 

 honeycombed tissue 

 that we found in the ***' 18 ' 



I. THE PERIOSTEUM. 



ends of the long bone. 



3. Bones are well supplied with blood-vessels and 

 nerves. We can find in every bone one or more holes 

 through which a small blood-vessel passes to the interior. 

 A living bone is covered with a membrane called the peri- 

 osteum. In this membrane is a fine net- work of blood- 

 vessels, from which a countless number of little vessels 

 pass directly into canals in the bone. These are called 

 Haversian canals. They run through every part of the 

 bone. 



