PROPERTIES OF THE HEART MUSCLE. 571 



mines in part the size of the ventricular cavity (the diastolic 

 volume) and the extent of the charge it will take from the veins 

 and auricles. As will be described in the next chapter the tone 

 of the heart muscle is dependent in part upon its extrinsic nerves, 

 but it is more dependent probably upon the composition of the 

 blood. Like the property of rhythmicity, that of tonicity is most 

 developed at the venous end of the heart. At least this is the case 

 with the heart of the cold-blooded animals, upon which this 

 property has been studied most carefully. The ventricle of the 

 terrapin, or strips excised from the ventricle and suspended so that 

 their movements can be recorded, often vary greatly in length 

 with differences in condition. These variations are due to changes 

 in tone. Not infrequently these changes take on a rhythmical 

 character; so that if the ventricle is beating one sees upon the 

 record regular tone waves, an alternate slow shortening and slow 

 relaxation quite independent of the rhythmical beats. The tissue 



Fig. 238.— To show tone waves in heart n.uscle. 'i'he record shows contractions of . 

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

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

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



of the auricle and especially of the sinus venosus exhibits this 

 property to a much more marked extent (see Fig. 238) . 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 often- 

 times in a rhythmical way. Fano* has made a special study of 

 this property and has suggested that the tone changes or contrac- 

 tions may be due to the activity of a substance in the heart dif- 

 ferent 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 to the undifferentiated sarcoplasm. 



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

 gewid., Leipzig, 1887. 



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



