28 



THE INDIVIDUAL ORGANISM 



into a single framework by a variety of devices. These include dovetailed 

 connections that lock certain bones immovably together, cartilaginous 

 junctures or pads that provide slightly flexible unions between bones, and 

 joints that are more or less freely movable. 



Skeletal units. The most conspicuous units of the skeleton are obvi- 

 ously the bones. These may be roughly classified according to shape. There 



schium 



matrix 



outer 

 lamellae 



cartilage cells 



foramen of blood vessel 



Fig. 3.1. Diagrammatic section through the upper end of the femur, with magnified detail 

 of (A) spongy bone, (B) cross section of compact bone of the shaft, and (C) articular 

 cartilage. 



are the long bones, including those of the upper and lower arms and legs; 

 short bones, such as those of the wrist and ankle ; flat bones, such as the 

 shoidder blade and some of the cranial bones; and irregular bones, exempli- 

 fied by the vertebrae. 



If a long bone is sawed lengthwise and the cut surface is examined, 

 the following parts can easily be identified. There is first a thick, hard 

 outer layer or shell of compact bony material. This forms a cylinder sur- 



