THE SUPPORTING TISSUES. 93 



there is a rather strong fibrous membrane completely invest- 

 ing it except at the ends, called the periosteum. This is a 

 membrane mostly of white fibres and some yellow elastic 

 tissue, which serves to carry the blood-vessels which are to 

 enter and nourish the bone. In the meshes of this perios- 

 teum the entering blood-vessels divide and branch, and so 

 plunge into the bone at many different points. * Immedi- 

 ately under the periosteum is a layer of ordinary osteoblasts. 

 The connection of these osteoblasts with the formation of 

 bone is mentioned further on. 



The current notion that the periosteum is the thing that 

 nourishes the bone and even produces it is correct only in 

 so far as it carries and distributes the blood-vessels which 

 enter the bone, and has under it the bone corpuscles which 

 form new layers of bone. For this reason, if in any surgical 

 operation or otherwise the periosteum is removed from the 

 bone, the bone soon dies for lack of nourishment. The 

 'ability of the periosteum with the osteoblasts underneath it 

 to form bone is strikingly illustrated in instances where 

 such periosteum removed from a bone was tunneled in 

 among muscles and in that position gradually developed a 

 new bone. But it must be remembered that this formation 

 of new bone was in no part a function of the periosteum 

 itself, which is a mere connective tissue membrane, nor 

 even of the blood-vessels which it carries in numbers, but 

 of the osteoblasts included under it. 



If some of the cancellated bone from the ends be exam- 

 ined in a similar way under a microscope as the section 

 taken from a shaft, the view is quite similar, except that 

 the Haversian systems are a little larger and their arrange- 

 ment not so compact. 



ORIGIN AND GROWTH OF BONE. 



In the way in which bones originate in the body they 

 are divided into two classes, called the membrane bones 

 and the cartilage bones. Membrane bones are those bones 

 which have never been preceded by cartilage, but have 



