THE ARTICULATED SKELETON 79 



Their usefulness is due to their physical characteristics for in- 

 stance, the hardness of bones enables the framework which they com- 

 pose to support the soft parts of the body, and in certain localities 

 enables them to protect internal organs. An important example 

 is the neural canal with its contents' the brain and spinal cord. 



Again, it is this same quality of hardness which enables the skel- 

 eton to bear direct pressure and the body weight. Osseous tissue 

 in certain bones notably the femur and the os coxae is especially 

 arranged in lines of pressure for this purpose; namely, that super- 

 imposed weight may be borne with the least strain upon the bone. 



The relation between the shapes of bones and the arrangement 

 of their two tissues has a direct bearing upon their usefulness and 

 the convenience with which it is exercised. Examples are seen 

 in the long bones their (comparatively) large extremities enter 

 into the formation of joints; they also give attachment to many 

 muscles which move the joints. Here, extent of surface is needed 

 and cancellous bone is used with but a thin covering of compact, 

 thus securing the necessary surface without undue weight. Their 

 shafts give attachment to fewer muscles, but their position in the 

 extremity exposes them to violence (applied transversely) and 

 calls for endurance of strain. Hence, for these two reasons first, 

 that extent of surface is unnecessary; and second, that strength 

 and endurance are demanded' the compact tissue is appropriate. 

 It also secures a convenient slenderness of bone where the bulk of 

 muscle tissue is greatest. 



By far the greatest variety of functions is seen in the articu- 

 lated skeleton, whereby the movements necessary to the well-being 

 of the individual are made possible by the character of the joints. 



The movements of the trunk are Iimited 5 but sufficient for the 

 needs of the organs which it contains; while those of the extremities 

 are many and free. They may resist external force; they may 

 themselves overcome opposing forces. They may be used as 

 weapons of offense or defense. Facilities for transporting the body 

 from place to place, or locomotion, are provided by the articulated 

 bones of the lower extremities; and the power of the upper ex- 

 tremities to perform a thousand necessary acts would not exist 

 without a similar framework. These points have been mentioned 

 already, and will be dwelt upon later in connection with the study 

 of the muscular system. 



