BONES OF THE HEAD 59 



The Haversian canals are occupied by bloodvessels, 

 nerves, and lymphatics, except in the region of the 

 heads of long bones; owing to their absence this por- 

 tion of the bone is better enabled to withstand the 

 pressure imposed upon it. The above arrangement 

 of canals is termed the Haversian System. 



The medullary cavity is a large space within the 

 centre of the long bones. It is lined by a fibrous layer, 

 the endosteum, and contains the nutrient marrow. 



Marrow is of two varieties, red and yellow. The 

 former color is seen in healthy, young individuals, the 

 latter occurs in those beyond the prime of life. The 

 presence of a great amount of fat causes the latter to 

 assume its yellow color. The cellular elements are few 

 or are wanting. In disease it may become red. It is 

 derived from the endosteum between the compact bone 

 and the medullary cavity, and consists of a delicate 

 frame-work of reticulum holding a compact capillary 

 plexus and cells. These cells are classified as: Marrow 

 cells or myelocytes, nucleated red blood cells or erythro- 

 blasts, white blood cells or leukocytes, myeloplaxes. (For 

 a better knowledge of these cells the reader is referred 

 to the standard works on histology or hematology.) 



The function of red marrow is to produce erythro- 

 cytes, granular leukocytes, and to store fat. Bones 

 are nourished by an artery which enters the nutrient 

 foramen, seen on the surface of bones, and by branch- 

 ing into smaller vessels, called capillaries, pass into 

 the Haversian canals;, other vessels enter Volkmann's 

 canals to nourish the most external lamellae beneath 

 the periosteum. 



BONES OF THE HEAD 



The skull is divided into two parts: the cranium 

 and face; the former lodges and protects the brain 

 and its membranes, bloodvessels, and nerves; the 



