THE SKELETON 221 



1. The cells in the surface layer of the bone receive 

 lymph from the capillaries in the periosteum. 1 It gets to 

 them through the short canaliculi that run out to the surface. 



2. The cells within the interior of the bone receive their 

 nourishment from the small blood vessels in the Haversian 

 canals. Lymph from these vessels is conveyed to the cells 

 through the canaliculi that connect with the Haversian 

 canals. 



Plan and Purpose of the Skeleton. The framework of 

 the body is such as to adapt it to a movable structure. 

 Obviously the different parts of the body cannot be secured 

 to a foundation, as are those of a stationary building, but 

 must be arranged after a plan that is conducive to motion. 

 A moving structure, as a wagon or a bicycle, has within it 

 some strong central part to which the remainder is joined. 

 The same is true of the skeleton. That part to which the 

 others are attached is a long, bony axis, known as the 

 spinal column. Certain parts, as the ribs and the skull, are 

 attached directly to the spinal column, while others are at- 

 tached indirectly to it. The arrangement of all the parts 

 is such that the spinal column is made the central, cohering 

 portion of the skeleton and also of the whole body. 



Besides the general arrangement of the parts of the 

 skeleton, there is such a grouping of the bones in each of 

 its main divisions as will enable them to serve definite pur- 

 poses. In most places they form mechanical devices 

 for supplying special movements, and in certain places 

 they provide for the support or protection of important 

 organs. In most cases there is a definite combination of 

 different bones, forming what is called the bone group. 



l The dependence of the. outer layers of hone cells upon the periosteum for 

 nourishment causes a destruction of this membrane to affect seriously the bone 

 beneath, producing in many instances a decay of the bone substance. 



