Varieties of muscle Structurally we may distinguish three main types of muscle each possessing certain quite definite histological and functional properties of its own. Within one type, however, rather marked deviations, both structurally and functionally, are found. These undoubtedly are to be associated with the varied responses commonly found in muscle of one particular type in different organs or parts of the body or even within the same organ or structure. Good examples of this are present in the small intestine and heart. The smooth muscle of the duodenum has a different periodicity in all phases of its contraction from that found in the lower ileum, that of the former being more active. The atria of the heart are more irritable and possess a greater rhythmicity than the ventricles. In studying the physiology of muscular activity, it is essential that an understanding of the general structural characteristics and properties of each of these three types be obtained since no form of activity can take place without involving one, two or all three types. Thus, when one begins to run, the most obvious response is that of the skeletal muscles, but contemporaneously with the increased exertion there is in addition an automatic and involuntary adjustment of the circulatory system including the heart, arteries, veins, and capillaries, of the respiratory apparatus, and of other organs and systems. All these are an attempt to supply the increased demand of the tissues for oxygen.		Home Classes of levers Varieties of muscle Smooth muscle The blood supply to muscles The neurone Cardiac muscle On the nomenclature of skeletal muscles The nerve supply to muscles Structure of a skeletal muscle Central nervous system The reflex arc and reflex action Physiologic properties of muscle Mechanical Changes The effects of temperature on muscular contraction Muscle glycogen, its source and fate The reaction of muscles Oxygen consumption of muscle Determination of the oxygen consumption of muscle The viscosity factor in muscle and muscle efficiency Significance of the recovery heat Production of Electricity