MINUTE STRUCTURE OF BONE. 



255 



or less degree, and that the difference between the two varieties of tissue depends 

 on the different amount of solid matter compared with the size and number of the 

 open spaces in each ; the cavities being very small in the compact parts of the bone, 

 with much dense matter between them ; whilst in the cancellated texture the spaces 

 are large, and the intervening bony partitions thin and slender. There is, accord- 

 ingly, no abrupt limit between the two, they pass into one another by degrees, the 

 cavities of the compact tissue widening out, and the reticulations of the cancellated 



Pig. 295. LONGITUDINAL SECTION THROUGH THE UPPER END OF THE FEMUR SHOWING THE CANCELLODS 



STRUCTURE OP THE HEAD AND THE COMPACT SUBSTANCE OP THE SHAFT. (From a photograph by 



Zaaijer. ) 



becoming closer as they approach the parts where the transition between the two 

 takes place. 



In all bones, the part next the surface consists of compact substance, which forms 

 an outer shell or crust, whilst the spongy texture is contained within. In a long bone, 

 the large round ends are made up of spongy tissue, with only a thin coating of com- 

 pact substance (fig. 295) ; in the hollow shaft, on the other hand, the spongy texture 

 is scanty, and the sides are chiefly formed of compact bone, which increases in 

 thickness from the extremities towards the middle, at which point the girth of the 

 bone is least, and the strain on it greatest. In tabular bones, such as those of the 

 skull, the compact tissue forms two plates, or tables, as they are called, inclosing 

 between them the spongy texture, which in such bones is usually named diploe. 

 The short bones, like the ends of the long, are spongy throughout, save at their 



