78 CONNECTIVE TISSUE CAETILAGE BONlJ 



difficulty distinguished from lymphocytes except when characteristically 

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

 ules. They become the bone cells of compact bone. Osteoblasts and lympho- 

 cytes are genetically closely related., both being relatively slightly differen- 

 tiated 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 the cells by whose 

 agency bone is destroyed during the processes of development and growth. 

 They are similar to, but not identical with, the polykaryocytes of hemo- 

 genic foci which are concerned with the process of blood-cell formation. 

 The osteoclasts originate by a process of fusion of reticular cells of the 

 marrow; the hemogenic polykaryocytes originate from lymphocytes 

 (hemoblasts) by repeated amitotic division of the nucleus. 



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

 supplied with blood. The nutrient or medullary artery penetrates 

 obliquely 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. Certain of the ter- 

 minal arteries anastomose with those of the cancellous epiphyses, and 

 with the arteries which enter the Haversian canals of the compact bone 

 from the periosteum. 



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 



