FORMATION OF BONE. 83 



FORMATION AND GROWTH OF BONE. 



In the earlier stages of animal growth, there is form- 

 ed in those parts of the system which are ultimately to 

 be supplied with bone, a cartilaginous pattern in minia- 

 ture, 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 towards the circumference, or extremi- 

 ties, until the whole becomes ossified. 



In the cylindrical bones the process begins in the mid- 

 dle of the cylinder, forming a ring there, which increas- 

 es in both directions, until the whole becomes bone. 

 Several of the bones of animals, particularly those ol 

 the skull, are not completely formed until the animal 

 is of considerable age. 



The heads of the bones are formed independently of 

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

 tilage between them. Afterwards 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. 



The bones are well supplied with blood vessels, which 

 enter about the middle of the long bones, and penetra- 

 ting the central cavity, pass both upwards and down- 

 wards, 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. 



Every bone is surrounded by a thin membrane, call- 

 ed the periosteum, from which pass into the exter- 

 nal, and most solid part of the bone, thousands of fine 

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



In what manner is it said bones are formed 7 Are the long bones of 

 young animals formed of one, or several pieces 1 Are the solid parts of 

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

 surrounds the bones called 1 What vessels deposite, or secrete bone ? 



