104 LOCOMOTION. [CHAP.V. 



The shaft is never perfectly cylindrical : although in some bones 

 it approaches that form, in others it is prismatic. It is hollow, as 

 already mentioned, and contains medulla. This arrangement has 

 the advantage of making the bone very much lighter than it would 

 have been if solid; while it is attended with no sacrifice of strength, 

 since the central osseous substance is that which contributes least 

 to its power of resistance. 



The strength of the shaft is amply provided for by its being com- 

 posed of compact tissue, of thickness proportionate to the length of 

 the bone, and the bore of the medullary canal. In the curved bones, 

 additional strength is obtained in the position where the bone would 

 be most likely to yield, by increased thickness and density along its 

 concavity. Of this provision a good example will be found in the 

 spine of the femur, a ridge of extremely dense bone, placed along 

 its posterior concave surface. In the bent bones of rickety sub- 

 jects which have become fully ossified, the compact tissue on the 

 concavity of the bend acquires an enormous development. 



At the extremities of the long bones the medullary canal ceases; 

 the osseous tissue expands ; and the cancellated texture takes the 

 place of the compact substance of the shaft, and forms the whole 

 thickness of these portions of the bone, the medulla penetrating into 

 its cells. Here great strength is not required, but surface is needed 

 for the articulation of the bones together, and for affording attach- 

 ment to ligaments and tendons. The cancellated tissue is admirably 

 adapted to attain this object; for, while by the looseness of its tex- 

 ture it readily affords an extent of surface, its lightness is such, that 

 even a considerable bulk of it does not materially affect the weight 

 of the bone. The surface of this texture is covered with a thin 

 cortex of compact tissue, which is perforated by innumerable orifices 

 for the transmission of vessels. 



The long bones are the great levers of the extremities ; as the 

 bones of the thigh and leg, arm and fore-arm. Among the bones of 

 the hand and foot are certain ones which have all the anatomical 

 characters of the long bones, except that of length ; they may, 

 therefore, be grouped together in a class under the name of short 

 bones. These are, the metacarpal and metatarsal bones, and the 

 phalanges of the fingers and toes. 



The flat bones are remarkable for their slight thickness ; they are 

 composed of two thin layers of compact tissue, enclosing a layer of 

 cancellated texture of variable thickness. Examples of this class 

 of bones may be found in most of those enclosing the great cavities 

 of the body ; as the bones of the cranium, the ribs, the scapula, the 



