CHAPTER III 



THE FRAMEWORK OF THE BODY 



The form and symmetry of the human body, as well as the greater part 

 of its bulk and weight, are determined by the skeletal framework and the 

 muscles and skin that cover it. It is the combination of skeleton and 

 muscle that performs all the voluntary movements, makes possible 

 locomotion, and, with the skin, forms a protection and support for nearly 

 all of the more delicate internal organs. Moreover, it is the muscles that 

 utilize most of the energy taken into the body, that provide the greater 

 part of the body heat, and that produce most of the waste that must be 

 continuously eliminated. 



THE LOCOMOTOR SYSTEM 



Because the skeletal system and the skeletal muscles are so intimately 

 related in their functioning, they are frequently considered together 

 under the heading locomotor system. A study of the structures involved 

 in a simple movement of the arm (Fig. 3.10) will illustrate the interde- 

 pendence between the skeleton and the muscles that surround and cover 

 it. 



The Skeleton 



Protoplasm, as we have seen, is largely made up of water. It tends to 

 keep a definite volume but cannot keep a definite shape unless it is held 

 in by some membrane or wall. In the instance of single cells and of the 

 smaller and simpler multicellular animals, the presence of cell membranes 

 or cell walls gives sufficient support. The larger and more highly developed 

 many-celled animals, however, especially those that must live upon land, 

 have had to develop a more or less complicated accessory supporting 

 mechanism. In the vertebrates, this mechanism consists of an internal 

 skeleton made up of bones, ligaments, and cartilages. The skeleton not 

 only helps to support the massive protoplasmic elements that constitute 

 the body proper but also provides leverage for the action of the muscles 

 that move the body. It is imperfectly represented by the usual mounted 

 skeleton or by the bones that remain after the rest of the body has dis- 

 integrated. In life, the 200 odd bones of the human skeleton are connected 



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