78 CONNECTIVE TISSUE CARTILAGE BONE 



difficulty distinguished from lymphocytes except when characteristically 

 arranged as a membranous coat upon the surface of bony walls or 

 spicules. They become the bone cells of compact bone. Osteoblasts and 

 lymphocytes are genetically closely related, both being relatively slightly 

 differentiated mesenchymal cells. In the bone marrow of the turtle 

 osteoblasts may differentiate into leukocytes. It seems probable that 

 persistent fetal osteoblasts of adult red marrow may function as parent 

 blood-cells. 



OSTEOCLASTS. These are giant multinuclear cells, often containing 

 as many as ten to twenty or more nuclei. They are believed to be the 

 cells by whose agency bone is destroyed during the processes of develop- 

 ment and growth. They are very similar to, perhaps identical with, the 

 polykaryocytes of hemogenic foci which are concerned with the process of 

 blood-cell formation. The genetic relationship of these marrow giant 

 cells to the leukocyte series and their full significance for the osteogenic 

 and hemogenic processes are not completely elucidated. 



Blood Supply. Marrow, and especially the red variety, is richly 

 supplied with blood. The nutrient or medullary artery penetrates ob- 

 liquely through the nutrient foramen to the marrow cavity of a long 

 bone where it divides into an ascending and descending branch and 

 supplies an abundance of small arteries to all portions of the medulla. 

 The terminal arteries end in broad capillary vessels whose wide lumen 

 and delicate endothelial walls determine their character as sinusoids. It 

 was formerly thought that the endothelial walls of these vessels were here 

 and there deficient, and although recent investigations discredit the 

 former observations, the all-important fact remains that the endothelial 

 walls are pervious to both red and white blood-cells. Neither is this the 

 only location where the red as well as the white cells may, under certain 

 conditions at least, penetrate the endothelial walls of the blood-capillaries. 



Efferent veins return the blood from the sinusoidal -capillaries of the 

 marrow. These veins, passing as companion veins to the medullary 

 artery through the nutrient foramen, or independently through separate 

 foramina, as also those of the bony tissue, are not supplied with valves. 

 Outside of the bones, however, these same veins contain abundant valves. 



The Lymphatics. The lymphatics of bone occur in great abundance 

 in the periosteum, and as perivascular spaces penetrate the canals of 

 Havers and Volkmann and thus reach the medullary cavity. The exist- 

 ence of lymphatics within the marrow, other than in the sheaths of the 

 blood-vessels, is doubtful. 



The Nerves. The nerves accompany the blood-vessels in all portions 



