286 



PHYSIOLOGY 



CHAP. 



vcs 



With the object of determining the limits of this regulatory 

 activity of the nervous system as exactly as possible, we shall in 

 the next two chapters discuss the better ascertained and more 

 important facts of human physiology, with respect to the physio- 

 logical conditions that underlie the activity of the heart and 

 vessels. 



I. As shown in Chapter VII., the mechanical activity of the 

 heart is due to the regular sequence of three different states in its 

 muscles contraction, expansion, and rest. In order to obtain a 

 clear idea of the origin, succession, and propagation of these three 

 states in the different segments of cardiac muscle, we must return 



to this subject and 

 attentively study 

 the exposed heart of 

 the living frog or 

 tortoise (Fig. 120). 



On cutting the 

 fraenuni, which at- 

 taches the posterior 

 wall of the ventricle 

 to the dorsal surface 

 of the pericardium, 

 and lifting up the 

 heart, we see that 

 the contraction com- 

 mences along the 

 venae cavae, and 

 spreads from these 

 to the sinus venosus 

 in which they unite. 

 From the 



II 



FIG. 120. 1 and 2, Anterior and posterior aspects of frog's heart. 

 At, Auricles ; V, ventricle ; A, A, rami of aorta ; Ba, aortic bulb ; 

 'So, sinus venosus; Vci, vena cava inferior; Ves, vena eava V6UOSUS, which Opens 

 superior ; Vp, pulmonary vein ; I7i, hepatic vein. 3 and U, 

 Anterior and posterior aspects of tortoise. heart. A, A', Pulinon- 

 ary arteries; Ji, ', left and right aorta; D, D', left and right 

 subclavian arteries; H, heim-anmihis of bulb; /, auriculo- 



into 

 p i p 4-i-. p 

 le ' 



, i i 



tlie I'lgllt 



Prmr ranrirm 



ventricular groove ; P, P', pulmonary veins ; M, L, left and right spreads tO the tWO 

 superior vena cava ; \, inferior vena cava ; R, coronary vein. " . ,, 



auricles ; from the 



two auricles it passes to the ventricle ; and from the ventricle, 

 lastly, to the bulbus arteriosus, which contracts actively in the 

 frog, as first noted by Spallanzani. The contraction or systole of 

 the heart thus takes the form of a wave, which originates in the 

 afferent vessels and passes to the heart, where it spreads peristaltic- 

 ally from auricles to ventricle, leaving the heart by the afferent 

 vessels. 



It should, however, be noted as a fact of great importance that 

 the peristaltic wave of contraction does not proceed uniformly, but 

 undergoes a delay or block at the junction between the sinus and 

 the auricles, the auricles and the ventricle, and, lastly, at that 



