IX 



CAEDIAC MUSCLE AND NEKVES 



309 



the ventricular, then those of the auricular portion. On sub- 

 stituting air or oxygen for the said gases, shortly after the beats 

 have been arrested, they are seen to reappear, first in the auricular, 

 subsequently in the ventricular section. In short, the embryonic 

 heart of the chick, which has no nerve elements, exhibits like the 

 adult heart a decrease of automaticity from the venous to the 

 arterial end (Fig. 136). 



(c) No less interesting are the observations of Fano in regard 

 to the mode in which the rhythm- 

 ical automatic activity of the 



same embryonic heart becomes 

 exhausted. When isolated and 

 exposed to favourable conditions 

 of moisture and temperature, it 

 continues to beat for a time which 

 usually exceeds one hour, and 

 may reach a maximum of two or, 

 three. But in the last stage 

 its life, rhythmical is transformed 

 into periodic activity, as repre- 

 sented by groups of beats separated 

 by long pauses, in complete agree- 

 ment with the phenomenon dis- 

 covered by us on the adult heart 

 of the frog. As in that, so in 

 the embryonic heart, the periodic 

 rhythm gradually resolves itself 

 into an irregular series of single 

 beats, which become constantly 

 weaker and less frequent until 

 they vanish altogether. The 

 auricular or venous segment of 

 the heart not only beats a good 

 deal longer than the ventricular 

 segment, but it exhibits the 

 periodic grouping of beats much 

 later. After the automatic rhythm 

 ceases, reflex excitability continues for some time. Since these 

 effects appear in a part entirely devoid of nervous elements, they 

 can only be due to automatic or reflex excitability of the embryonic 

 muscle cells. 



(d) On repeating the same observations and experiments on 

 the chick's heart in the second half of its development, i.e. on the 

 eleventh day of incubation, when there is no essential morphological 

 difference between it and the adult heart of vertebrates, Bottazzi 

 obtained the same results as those recorded by Fano for the first- 

 hours of incubation. In that stage of development also automaticity - 



FIG. 13(3. Chick's heart on third day of in- 

 cubation. (Fano.) AD, venous extremity, 

 from which the auricles develop ; BC, 

 arterial end, from which the bulbi arteriosi 

 develop ; EF, median line of ventricular 

 portion ; AB, concave line, or lesser curva- 

 ture of heart ; DFC, convex line, or greater 

 curvature of heart. Each of the divisions 

 indicated below the figure corresponds to 

 0'05 mm., and the whole line to 1 mm. 



