STEUOTURE OE A LONG BONE 



237 



Structure of a long bone. — (See Ex. XLIX., b.) A 

 leg of lamb or a soup bone furnishes a good example for 

 the study of such a bone. Examination 

 shows the following external features; 

 At each end is an enlargement called 

 the head, connected by a relatively nar- 

 row, cylindrical shaft. The enlarged 

 heads are usually more or less roughened 

 except at the very ends to allow the 

 attachment of muscles, while the ends 

 are coated with layers of cartilage to 

 form a smooth articulating surface where 

 the bone is joined to another bone. 

 The central shaft is smooth and smaller 

 in diameter than the ends, and usually 

 functions mainly for support. Very few 

 muscles are attached to this part. All 

 parts, except those covered with carti- 

 lage, are encased in a thin membrane 

 (the periosteum). This periosteum is a 

 layer of thin, tough, connective tissue 

 which distributes blood and through 

 whose agency most of the new bone is 

 formed. 



If we cut this bone lengthwise, we 

 find the following internal structure: 

 The two heads are filled with a spongy 

 mass of bone tissue. Over the whole 

 outside is a thin layer of very hard, 

 compact bone, thinnest on the heads, 

 and thickening to form the main part 

 of the shaft walls. In the center of the shaft is a cavity 



Fig. 95 — a long bone 

 split length-wise fa, 

 liead ; b, shaft ; c, 

 sponge bone and 

 marrow. 



