180 THE CARDIAC MUSCLES. [BOOK i. 



The Mechanism of the Normal Beat. 



The cardiac Muscles. When a frog's heart which has ceased 

 to beat spontaneously is stimulated by touching it with a blunt 

 needle, a beat is frequently called forth ; this artificial beat differs 

 in no obvious characters from a natural beat. The latent period of 

 such an artificial beat is remarkably long, the length varying within 

 very wide limits. Thus the cardiac contraction is more like that 

 of an unstriated than of a striated muscle. The beat is in fact a 

 modified or peculiar form of peristaltic contraction. In the hearts 

 of some animals, the ventricle forms a straight tube ; and in these 

 the peristaltic character of the beat is obvious ; but in a twisted 

 tube like that of the vertebrate ventricle, ordinary peristaltic 

 action would be impotent to drive the blood onward, and is 

 accordingly so far modified that the peristaltic character of the beat 

 is recognised only when the action of the heart becomes slow and 

 feeble. 



The cardiac, like the skeletal muscular fibre, after a contraction 

 returns by relaxation to its previous shape, and the whole ventricle 

 (or whole heart) regains after a beat the form natural to its qui- 

 escent state. This diastolic expansion, though increased by, is not 

 dependent on, the influx of fluid into the cavities of the heart. 

 Thus the cavity of the empty quiescent mammalian left ventricle, 

 though smaller than when it is distended with blood as in its 

 normal action, is larger than when it is in systole or when rigor 

 mortis has set in ; moreover if its dimensions be artificially less- 

 ened, as when it is squeezed with the hand, it returns by an elastic 

 reaction to its former volume when the pressure is removed. 



The cardiac muscles in a healthy condition are, like the skeletal 

 muscles, very elastic. Their elasticity is however soon interfered 

 with by imperfect nutrition ; and a 'contraction remainder' (p. 57) 

 under certain circumstances is readily developed. 



Under the influences of certain poisons, veratrin, digitalin. &c., 

 the length of the beat is enormously prolonged, and the ventricle 

 is eventually thrown into a remarkable contracted condition, the 

 exact nature of which is perhaps not thoroughly understood, 

 though it is believed by many to be due to a deficiency of elastic 

 reaction. 



One great feature of the cardiac beat produced by artificial 

 stimulation is the absence of that relationship between the strength 

 of the stimulus employed and the amount of contraction evoked 

 which is so striking in a skeletal muscle (p. 85). The beat with 

 which a heart responds to a stimulus, e.g. a single induction shock, 

 is, if there be any response at all, equally large when a feeble as 

 when a strong stimulus is used, though the strength of the beat 

 evoked either by a strong or a weak stimulus may vary con- 

 siderably within even a very short period of time. 



