A text-book of physiology for medical students and physicians . sition of the the property of rhythmicity, that of tonicity is mostdeveloped at the venous end of the heart. At least this is thecase with the heart of the cold-blooded animals, upon whichthis property has been studied most carefully. The ventricleof the terrapin, or strips excised from the ventricle and susTpended so that their movements can be recorded, often varygreatly in length with differences in condition. These varia-tions are clue to changes in tone. Not infrequently thesechanges take on a rhythmical character;
A text-book of physiology for medical students and physicians . sition of the the property of rhythmicity, that of tonicity is mostdeveloped at the venous end of the heart. At least this is thecase with the heart of the cold-blooded animals, upon whichthis property has been studied most carefully. The ventricleof the terrapin, or strips excised from the ventricle and susTpended so that their movements can be recorded, often varygreatly in length with differences in condition. These varia-tions are clue to changes in tone. Not infrequently thesechanges 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 andespecially of the sinus venosus exhibits this property to a muchmore marked extent (see Fig. 238). The tone—that is, the lengthof the piece—if in strips, or the capacity of the chamber, if usedentire, is continually changing and oftentimes in a rhythmical. _ Fig. 238.—To show tone waves in heart muscle. The record shows contractions of astrip of the sinus venosus (terrapins heart) suspended in a bath of blood-serum. In addi-tion to the sharp contractions marked by the lines there are longer, wave-like shorteningsand relaxations, irregular in character, which are due to variations in tone. way. Fano* has made a special study of this property and hassuggested that the tone changes or contractions may be due tothe activity of a substance in the heart different from that whichmediates the ordinary contractions. Botazzif suggests that, whilethe usual sharp systolic contraction is due to the cross-striated(anisotropous) substance, the slower tone changes may be due * Fano, Beitrage zur Physiologie. C. Ludwig, zu s. 70 Geburtstagegewid. Leipzig, 1887. t Journal of Physiology, 21, 1, 1897. 572 CIRCULATION OF BLOOD AND LYMPH. to tke undifferentiated sarcoplasm. However this may be
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