FORMATION OF BONE. 79 



FORMATION AND GROWTH OF BONE. 



220. IN the earlier stages of animal growth, there is 

 formed in those parts of the system which are ultimately to 

 be supplied with bone, a cartilaginous pattern in miniature 

 of the bone itself. This cartilage is semi-hard, somewhat 

 tenacious and translucent. When the bone begins actually 

 to form, the cartilage becomes absorbed at the centre of 

 the piece, and a few ossific particles are deposited in its 

 place. As the process goes on, cartilage continues to be 

 taken up, and bone formed in its room from the centre 

 toward the circumference, or extremities, until the whole 

 becomes ossified. 



221. In the cylindrical bones the process begins in the 

 middle of the cylinder, forming a ring there, which in- 

 creases in both directions, until the whole becomes bone. 

 Several of the bones of animals, particularly those of the 

 scull, are not completely formed until the animal is of con- 

 siderable age. 



222. The heads of the bones are formed independently 

 of the shafts, being separate pieces with a thin layer of 

 cartilage between them. Afterward these parts unite, the 

 cartilage being replaced by bone ; but this does not 

 happen in our species until the age of fifteen or eighteen 

 'years. 



223. The bones are well supplied with blood-vessels, 

 which enter about the middle of the long bones, and pene- 

 trating the central cavity, pass both upward and down- 

 ward, supplying the substance of the bone with small 

 branches, and giving off some very delicate arteries which 

 secrete the marrow. It is the arteries which thus pass into 

 the bones, giving off the most delicate fibres through every 

 part of its substance, that secrete and form the bone itself. 



224. Every bone is surrounded by a thin membrane, 

 called the periosteum*, from which pass into the external, 

 and most solid part of the bone, thousands of fine blood- 

 vessels by which this part was formed, and is still nourished, 



In what manner is it said bones are formed ? Are the long bones of 

 young animals formed of one, or several pieces? Are the solid parts ol 

 bones supplied with blood-vessels or not ? What is the membrane which 

 surrounds the bones called ? 'What vessels deposite or secrete bone 7 



