44 THE INDIVIDUAL ORGANISM 



the oxidation of the simple sugar glucose. This substance and oxygen 

 enter the cell from the blood. Much of the sugar is not burned at once, 

 but is changed within the cell into glycogen (animal starch) and stored in 

 this form. This glycogen constitutes an energy reserve which can be 

 drawn upon during periods of muscular activity. When it is used, it is 

 broken down by a series of steps into lactic acid. Under conditions of 

 moderate muscular activity and sufficient oxygen supply all of the lactic 

 acid is used within the cell, some being burned for energy and some being 

 resynthesized into glycogen. When muscular action is prolonged or 

 excessively vigorous, insufficient oxygen may reach the muscle cell to 

 maintain this balance and oxidize the required amount of fuel. Under 

 such circumstances the glycogen-to-lactic acid reaction may itself 

 serve as an anaerobic (oxygenless) source of the energy required for 

 continued muscular activity. Excess lactic acid accumulates in the cell, 

 causing fatigue and building up an ''oxygen debt" in the form of un- 

 oxidized lactic acid. Some of the excess lactic acid is set free into the 

 blood, and is excreted via the kidneys; the remainder is in part oxidized 

 and in part rebuilt into glycogen during a period of rest and recovery from 

 fatigue. 



This explanation of the changes known to occur in the muscle cell 

 during and after contraction is very much simplified. Even so it will serve 

 to indicate how complex and how delicately balanced are the reactions 

 and processes involved in such an apparently simple act as "flexing the 

 biceps." 



The contraction of a whole muscle is produced by the shortening of its 

 individual cells and is thus many-cell-powered. It is initiated by nerve 

 impulses simultaneously reaching many individual cells, and these con- 

 tracting cells are held together and mechanically coordinated by connec- 

 tive tissues. The amount of muscular power exerted by a particular muscle 

 varies greatly. Thus the biceps may contract gently or powerfully. The 

 difference is largely a function of how many of the thousands of individual 

 cells are stimulated to contract. A gentle contraction utilizes only a part 

 of the whole number; when all of them contract together, the maximum 

 power of the muscle is exerted. Each muscle cell (muscle fiber) operates 

 on the principle of "all or none" ; i.e., it either contracts fully or not at all. 



Under the influence of continued exercise the skeletal muscles increase 

 in size and correspondingly in strength. Contrary to what might be sup- 

 posed, this enlargement is due not to an increase in the number of muscle 

 cells but to an increase in the size of the individual cells. Since it is a 

 response to activity, and since the activity of the muscle cell is determined 

 by the frequency with which nerve impulses are received, it is natural 

 that reduction in the number of nerve impulses should have the opposite 

 effect. If, therefore, the efferent (outgoing) nerve to a muscle is destroyed 



