6 4 TEXT-BOOK OF PHYSIOLOGY. 



in the height of the contraction has been termed the staircase effect, or 

 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- 



FiG.26. SINGLE CONTRACTIONS OF THE GASTROCNEMIUS MUSCLE AT DIFFERENT TEMPERA- 

 TURES. Time tracing 200 per second. (Brodie.) 



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. 



FIG. 27. CONTRACTIONS OF A GASTROCNEMIUS MUSCLE 

 WITH DIFFERENT LOADS. (Brodie.) 



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

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

 physiologic maximum value is about 30 C. If the temperature 

 of the muscle falls to 20 C. there is a corresponding decline in activity, 

 as shown by an increase of the latent period, a decrease in the height 

 of curve i.e., in the shortening of the muscle an increase both in 

 the contraction and relaxation periods. As the temperature approaches 

 o C, the height of the curve again suddenly increases, indicating, for 

 some unknown reason, an increase in the irritability. This is, however, 

 scarcely a physiologic condition. At a temperature of 40 C. to 50 C. 



