88 



THE HUMAN BODY. 



a 



and rough patches on the shaft indicate places to which 



muscles of the arm were fixed. 



Internal Structure. If the bone be divided longitudi- 

 nally, it will be seen that its shaft is hol- 

 low, the space being known as the medul-1 

 lary cavity, and in the fresh bone filled/ 

 with marrow. Fig. 37 represents such a 

 longitudinal section. It will be seen that 

 the marrow cavity does not reach into 

 the articular extremities, but 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 tjie thickest, the spongy or 

 cancellated bone forming only a thin 

 stratum immediately -around the medul- 

 lary cavity. To the naked eye the can- 

 cellated bone appears made up of a 

 trellis-work of thin bony plates which in- 

 tersect in all directions and surround 

 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 magnify- 

 ing glass. In the spaces of the spongy 

 portion lies, during life, a substance 

 known as the red marrow, which is 

 quite different from the yellow fatty- 

 -c marrow lying in the central cavity of the- 

 shaft. 



>scopic Structure of Bone. The 

 shows that the compact bone 

 contains cavities and only differs from the 

 FIG. 37. -The humerus spongy portion in the fact that these 



bisected lengthwise, a, r &J 



marrow cavity; 6, hard are much smaller and the hard true bony 



bone; c. spongy bone; , n . n n 



d, articular cartilage, plates surrounding them much more nu- 

 merous in proportion than in the spongy 

 parts. If a thin transverse section of the shaft of the hu- 

 merus be examined (Fig. 38) with a microscope magnifying 



