PHYSIOLOGICAL ANATOMY OF THE BONES. 485 



are found applied to the internal surfaces of the bones. They are very irreg- 

 ular in size and form (measuring T ^g-ff to ^^ of an inch, or 20 to 100 /A in 

 diameter), are finely granular, and present two to twenty or thirty nuclei. 

 The nuclei are clear and ovoid and are generally provided with a distinct 

 nucleolus. The myeloplaxes are rendered pale by acetic acid, and the nuclei 

 are then brought distinctly into view. They are particularly abundant in 

 the red marrow. 



In addition to the anatomical elements just described, the marrow con- 

 tains a few very delicate bundles of connective tissue, most of which accom- 

 pany the blood-vessels. In the foetus the adipose vesicles are few or may 

 be absent ; but in the adult they are quite abundant, and in some bones they 

 seem to constitute the whole mass of the marrow. They do not differ ma- 

 terially from the fat-cells in other situations. Holding these different struct- 

 ures together, is a variable quantity of semi-transparent, amorphous or slightly 

 granular matter. 



The nutrient artery of the bones sends branches to the marrow, generally 

 two in number for the long bones, which are distributed between the various 

 anatomical elements and finally surround the fatty lobules and the fat- vesicles 

 with a delicate capillary plexus. The veins correspond to the arteries in 

 their distribution. The nerves follow the arteries and are lost when these 

 vessels no longer present a muscular coat. Nothing is known of the presence 

 of lymphatics in any part of the bones or in the periosteum. 



The chief physiological interest connected with the marrow of the bones 

 is in its relations to the formation of blood-corpuscles. This question has 

 already been discussed in connection with the development of the corpuscular 

 elements of the blood. 



Periosteum. In most of the bones the periosteum presents a single layer 

 of fibrous tissue, but in some of the long bones two or three layers may be 

 demonstrated. This membrane adheres to the bone but can generally be 

 separated without .much difficulty. It covers the bones completely, except 

 at the articular surfaces, where its place is supplied by cartilaginous incrusta- 

 tion. It is composed mainly of ordinary fibrous tissue with small elastic 

 fibres, blood-vessels, nerves and a few adipose vesicles. 



The arterial branches ramifying in the periosteum are quite abundant, 

 forming a close, anastomosing plexus, which sends small branches into the 

 bony substance. There is nothing peculiar in the arrangement of the veins. 

 The distribution of the veins in the bony substance itself has been very little 

 studied. 



The nerves of the periosteum are very abundant and form in its substance 

 quite a close plexus. 



The adipose tissue is very variable in quantity. In some parts it forms a 

 continuous sheet, and in others the vesicles are scattered here and there in 

 the substance of the membrane. 



The importance of the periosteum to the nutrition and regeneration of 

 the bones is very great. Instances are on record where bones have been 

 removed, leaving the periosteum, and in which the entire bone has been 



