6o 



PHYSIOLOGY l-'OR BEGINNERS 



seen to be hollow, the bone enclosing a 

 cavity, the medullary cavity. This 

 cavity contains the marrow or medulla, a 

 vascular and therefore reddish tissue con- 

 taining much fat. 



If the bone be sawn longitudinally, the 

 medullary cavity will be found to extend 

 along the whole length of the shaft and 

 down to the enlarged ends, but not into the 

 enlargements. The bone at the ends is not 

 dense or compact as in the shaft, but spongy 

 or, as it is called, cancellous. The marrow 

 of the medullary cavity with the blood- 

 vessels in it extends into the cavities of the 

 spongy bone, and numerous blood-vessels 

 pass from the medulla into the compact 

 bone along the length of the shaft also. 

 The bone is thus well supplied with blood- 

 vessels, partly from the periosteum and 

 partly from the marrow. In the spongy 

 bone the vessels lie in the marrow in 

 the cavities of the sponge work ; in 

 the compact bone they lie in numerous 

 canals running longitudinally along the 

 shaft. These canals, called the Haversian 

 canals, which, although they run mostly 

 longitudinally, are connected with one an- 

 other, open here and there inwards into the 

 k liliJIrav medullary cavity and outwards on to the 



surface of the bone, and at these openings 

 blood-vessels enter them from the medulla 

 or from the periosteum. Some bones have 

 no medullary cavity, being merely cancellous 

 or spongy within ; such are the ribs, 



FlG< 26 le^h e wfc e nur Ut vertebras > and some of the smaller bones. 



a, the upper end consist- Although the enlargements at the ends of 

 ing of spongy bone ; i>, a \ on g bone are loose or spongy in texture 



the marrow cavity ; c, t i / / 



the spongy bone of the they are really as strong as the shaft, for 

 pSRSitftifrS. the various sheets and spicules of bone 

 left unshaded in the making up the sponge work run in 



figure. 



