G8 



THE CONNECriVE TISSUES 



osteocytes in livino; bone. (Fig- 39.) These cells secure their 

 nutrition from the Haversian canal which carries the vascular tissue. 

 There is one main nutrient foramen near the middle of the shaft 

 which carries the medullary artery from the periosteum into the 

 central marrow cavity and at the epiphyses there are several 

 smaller foramina. Within the matrix of the bone there are numerous 

 collagenous fibers extending in different directions in adjacent 

 lamellte, thereby increasing the strength of the shaft. In the central 

 marrow cavity the same marrow elements are present as in the 

 epiphyses but there is an increasing accumulation of fat cells in 

 this tissue with advancing age and loss of blood-forming activity. 



Fig. .39. — Photograph of a portion of a cross-sention of an Haversian system 

 showing the lacuntp and canaUculi occupied by the osteocytes in Uving bone. The 

 alternating dark and Hght lamelhe indicate differences in their fibrous organization. 



Marrow. — In young animals, medullary spaces in bone are filled 

 with red marrow. This is composed of very loose fibroelastic ct)n- 

 nective tissue cells, reticular fibers and cells, nerve fibers, a few fat 

 cells, medullary sinusoids, a great variety of primiti^-e blood cells 

 and stages in blood cell formation, mature blood cells, and mega- 

 karyocytes. Since it is an hemapoietic center it is more fully ex- 

 plained under blood. 



With increasing age the marrow in the medullary cavity of the 

 shaft changes in character. The hemapoietic proi)erty is lost. A 

 great predominance of fat cells develops and since it has a light 

 yellow color it is known as yellow marrow. The marrow in the 

 epiphyses remains red. 



