STRUCTURE AND COMPOSITION OF BONE. JOINTS. 87 



glenoid cavity of the scapula; and on the lower are the simi- 

 lar surfaces, Cpl and Tr, which articulate with the radius and 

 ulna respectively. Besides carrying the articular surfaces, 

 each extremity presents several prominences. On the upper 

 are those marked Tmj and Tm (the greater and smaller tro~ 

 chanters), which give attachment to muscles; and similar 

 eminences, the external and internal condoles, El and Em, 

 are seen on the lower end. Besides these, several bony ridges 

 and rough patches on the shaft indicate places to which mus- 

 cles of the arm were fixed. 



Internal Structure. If the bone be divided longitudinally, 

 it will be seen that its shaft is hollow, the space being known 

 as the medullary cavity, and in the fresh bone filled with 

 marrow. Fig. 40 represents such a longitudinal section. It 

 will be seen that the marrow-cavity does not reach into the ar- 

 ticular extremities, but that there the bone has a loose spongy 

 texture, except a thin layer on the surface. In the shaft, on 

 the other hand, the outer compact layer is much the thicker, 

 the spongy or cancellated bone forming only a thin stratum 

 immediately around the medullary cavity. To the naked 

 eye the cancellated bone appears made up of a trellis-work of 

 thin bony plates which intersect in all directions and sur- 

 round cavities rather larger than the head of an ordinary 

 pin; the compact bone, on the contrary, appears to have no 

 cavities in it until it is examined with a magnifying-glass. 

 In the spaces of the spongy portion lies, during life, a sub- 

 stance known as the red marroiv, which is quite different from 

 the yellow fatty marrow lying in the central cavity of the 

 shaft. 



Microscopic Structure of Bone. The microscope shows 

 that the compact bone contains cavities and only differs from 

 the spongy portion in the fact that these are much smaller, 

 and the hard true bony plates surrounding them much more 

 numerous in proportion than in the spongy parts. If a 

 thin transverse section of the shaft of the humerus be 

 examined (Fig. 41) with a microscope magnifying twenty 

 diameters, it will be seen that numerous openings exist all 

 over the compact parts of the section and gradually become 

 larger as this passes into the cancellated part, next the medul- 

 lary cavity. These openings are the cross-sections of tubes 

 known as the Haversian canals, which ramify all through the 

 bone, running mainly in the direction of its long axis, but 



