66 TEXT-BOOK OF PHYSIOLOGY 



as a stimulus the single induced current, it has been found that if the 

 strength of the current is progressively increased, the height of the con- 

 traction will correspondingly increase until a certain maximum height is 

 attained. Until quite recently it has been assumed that this graded 

 response of the muscle was the result of a graded increase in the 

 strength of the stimulus, that a minimal stimulus caused a minimal 

 shortening of all the fibers composing the muscle, and that a maximal 

 stimulus caused a maximal contraction of all the fibers. The investi- 

 gations of Keith Lucas and others have rendered this explanation 

 doubtful. These investigations have shown that the minimal 

 shortening of the muscle is the result of the maximal shortening of 

 but a few fibers, and that the maximal shortening is the result of 

 the maximal shortening of all the fibers. Between the minimal and 

 maximal shortening an ever increasing number of fibers is called into 

 action by the ever increasing strength of the stimuli. Here as in the 

 case of the cardiac muscle, the "all or none" law holds true, viz., that 

 if a muscle fiber contracts at all to a stimulus, it at once reaches its 

 maximal value. That the skeletal muscle as a whole does not so 

 respond is due to the fact that the excitation process is prevented by 

 the sarcolemma from passing to adjoining fibers, at least transversely, 

 a histologic condition not present in the cardiac muscle in which the 

 excitation process passes rapidly in all directions. 



It has also been shown that the rate at which the muscle is stimulated 

 with a given stimulus of uniform strength will influence the char- 

 acter of the contraction process. If the intervals between the successive 

 stimulations be such as permit the muscle to recover from the effects of 

 the contraction, it may contract as many as a thousand times without 

 showing any particular variation from the normal form; but if the 

 intervals are shorter than that just stated it is found that from the 

 beginning of the stimulation each succeeding contraction slightly exceeds 

 in height the preceding contraction, until a certain maximum is reached 

 and maintained, indicating that for some reason the irritability and the 

 energy of the contraction have been increased. This gradual increase 

 in the height of the contraction has been termed the staircase effect, t)r 

 the treppe. In the beginning of the period of stimulation there is some- 

 times observed a decrease in the height of the contraction following 

 several stimulations before the staircase effect develops, indicating 

 a temporary decrease in the irritability. These staircase contractions 

 have been observed in the muscle of both warm-blooded and cold- 

 blooded animals. The cause for this increase in irritability upon which 

 the effect depends is attributed to the presence of certain chemic sub- 

 stances in the muscle arising as a result of its katabolism, such as 

 carbon dioxid, mono-potassium phosphate, and paralactic acid. These 

 compounds, when present in small amounts or in larger amounts for a 

 short time, augment the action of the muscle and give rise to the treppe 

 effect. (Lee.) In time, however, if the stimulation be continued, the 

 irritability declines, the height of the contraction diminishes and 

 finally the muscle ceases to respond to any stimulus. 



2. Variations in the Temperature. The temperature at which all phases of 

 the contraction process, as represented by the myogram, attain their 



