530 CIRCULATION OF BLOOD AND LYMPH. 



especially of the sinus venosus exhibits this property to a much 

 more marked extent (see Fig. 218). The tone, that is, the length 

 of the piece, if in strips, or the capacity of the chamber, if used 

 entire, is continually changing and oftentimes in a rhythmical 

 way. Fano* has made a special study of this property and has 

 suggested that the tone changes or contractions may be due to 

 the activity of a substance in the heart different from that which 

 mediates the ordinary contractions. Botazzif suggests that, while 

 the usual sharp systolic contraction is due to the cross-striated 

 (anisotropous) substance, the slower tone changes may be due 



Fig. 218. To show tone waves in heart muscle. The record shows contractions of a 

 strip ofthe sinus venosus (terrapin's heart) suspended in a bath of blood-serum. In addi- 

 tion to the sharp contractions marked by the lines there are longer, wave-like shortenings 

 and relaxations, irregular in character, which are due to variations in tone. 



to the undifferentiated sarcoplasm. However this may be, the 

 property of tonicity is an important one in the physiology of the 

 heart and of the other visceral organs. Through it a certain tension 

 of the musculature is maintained, and the size of the cavities is 

 controlled. The property may be of special regulative value in 

 the large veins where they open into the auricles, but at present 

 we have little positive knowledge of the conditions that control 

 the tonicity, of the extent of its regulating action normally, or 

 of the extent of its derangement under pathological conditions. 



*Fano, "Beitrage zur Physiologie." C. Ludwig, zu s. 70 Geburtstage 

 gewid. Leipzig, 1887. 



t "Journal of Physiology," 21, 1, 1897. 



