584 CIRCULATION OF BLOOD AND LYMPH. 



b l^r 



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 sus- 

 pended so that their movements can be recorded, often vary 

 greatly in length with differences in condition. These varia- 

 tions are due to changes in tone. Not infrequently these 

 changes take on a rhythmical character; so that if the ven- 

 tricle is beating one sees upon the record regular tone waves, 

 an alternate slow shortening and slow relaxation quite inde- 

 pendent of the rhythmical beats. The tissue 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 oftentimes in a rhythmical 



Fig. 238. To show tone waves in heart muscle. The record shows contractions of > 

 trip of the anus 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. 



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 

 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 ten- 

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

 and, therefore, the output of the ventricles is controlled. A 

 diminution in tonicity constitutes an important factor in the 

 pathological condition known as acute dilatation of the heart. 



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

 gewid., Leipzig, 1887. 



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



