BONE 77 



the periosteum into and through the superficial lamella? ; these are known 

 as the perforating fibers of Sharpey. Similar fibers connect together the 

 concentric and interstitial lamellae. The perforating elastic fibers are 

 frequently surrounded by an envelope of fibrous connective tissue. 



Bone Marrow. Bone marrow consists of a variety of connective 

 tissue, largely reticular, which is rich in fat cells and blood-vessels and 

 which also contains osteogenic and hemogenic elements, the marrow 

 cells or myelocytes. According to the relative proportion of these ele- 

 ments marrow is said to present two types, the yellow and the red marrow. 

 The yellow marrow consists almost entirely of fat, with only occasional 

 bands of true marrow tissue. The red marrow contains very little fat, 

 bub is so abundantly supplied with blood and marrow cells as to closely 

 resemble a very vascular lymphoid tissue. The embryonic medulla of all 

 bones contains fetal red marrow, but in later life the larger masses in 

 the medulla of the shafts of the long bones are, in man, changed to the 

 yellow variety. The red marrow, however, persists in the epiphyses of 

 the long bones and in cancellous bone generally; it is especially charac- 

 teristic of the marrow cavities of the ribs, vertebrae, base of the skull, and 

 sternum. It is the source of supply of blood-cells in the adult. 



BED MARROW. Eed marrow consists of fibrous and reticular tissues 

 which are infiltrated by marrow cells and richly supplied with small 

 blood-vessels. The smaller veins possess exceedingly thin walls, readily 

 pervious to the blood-cells. The walls are so delicate that it becomes 

 very difficult to determine with certainty whether or not their eiidothe- 

 lium, as also that of the capillaries, may be occasionally absent, thus 

 placing the blood-stream in direct communication with the pulp of the 

 bone marrow. 



The hemogenic elements of marrow will be described under the 

 subject of blood development, where red marrow must again be con- 

 sidered. At this point it is only necessary to describe the osteogenic 

 elements. These are (1) the osteoblasts, or bone builders, and (2) the 

 osteoclasts, or bone destroyers. The osteogenic process as a whole is of 

 course dependent upon the blood, with all its hemal elements. 



OSTEOBLASTS. These are cells which may assume various shapes 

 depending upon their spatial relationship to the bony substance. When 

 free they are of round or slightly oval shape; lining the marrow cavity 

 or covering the bone as portions of the periosteum or applied to spicules 

 of cancellous bone they may become considerably flattened. The nucleus 

 is generally round or oval, deeply chromatic and granular. As spheroidal 

 cells they have an average diameter of about 8 microns. They are with 



