RENOVATION OF BONES. 149 



tion of the vital constituents, becomes necessary 

 In old age, again, when the wants of the system are 

 reversed, and when positive diminution of existing 

 masses is required to put the frame into harmony 

 with the shrunk muscles and feebler powers of life, 

 the absorbent vessels carry away more of the vital 

 matter, leaving chiefly the earth, which, being less 

 susceptible of change, requires scarcely any support 

 from within ; and hence the brittle and compact 

 hardness of the bones, and their little capability of 

 uniting when fracture happens at an advanced pe- 

 riod of life. 



At birth many of the bones are, properly speak- 

 ing, of a cartilaginous nature. As ossification ad- 

 vances, the cartilage is removed by the absorbents, 

 and its place supplied by a kind of cellular mem- 

 brane, in the interstices of which the earthy parti- 

 cles are deposited ; the two forming, by their union, 

 the homogeneous whole called Bone. Although, 

 therefore, it is to the softer material alone that vital 

 properties essentially belong, it is usual to speak of 

 the life, the vessels, and the nerves of bones, as if 

 life belonged equally to the earthy and animal por- 

 tions. This is correct enough in reality, because 

 the union between the earthy and animal tissues is 

 always the product of life ; and the parts thus united 

 are, to all intents and purposes, living parts. 



To carry on the processes of waste and renova- 

 tion, by which every living structure is distinguished, 

 all parts of the body are provided, first, with ar- 

 teries, conveying to them red or nutritive blood ; 

 secondly, with exhalants, by which the new matter 

 is deposited, and which are believed to be the minute 

 terminations of the arteries; thirdly, with veins 

 by which the blood is carried back to the heart ; 

 fourthly, with absorbent vessels, which take up and 

 carry away the waste particles to be thrown out of 

 the system ; and, lastly, with nerves to supply all 

 these vessels, and the organs on which they are dis- 

 N2 



