454 PRINCIPLES OF GENERAL PHYSIOLOGY 



Refractory Period. This can easily be detected in the heart muscle. If a 

 stimulus is put in at various points on the course of a previous contraction, 

 natural or excited by artificial stimulus, no effect is produced until a certain stage 

 is reached and it is found that, within limits, the stronger the stimulus, the earlier 

 is a second contraction capable of being excited ; as already mentioned, in the 

 very earliest part of this period no contraction can be produced by any stimulus 

 whatever. If any effect at all is produced, it is the maximum one that the tissue 

 is capable of giving at that stage of recovery (see Figs. 138 and 139)T. 



Summation of Contraction. Mines (1913, 1, p. 22) shows that, in the ventricle 

 of the Selachian fish, Torpedo, an artificial stimulus, at a short interval after a 

 normal beat, produces a greater response than the normal one, and that this 

 response may occur at so short an interval that the previous contraction has not 

 completely disappeared, so that superposition may occur. As the interval 

 increases, the height of the second contraction decreases, until, at the normal 

 interval between spontaneous beats, the normal height of contraction is given by 

 an artificial stimulus. These facts and the relatively short refractory period 

 associated with the phenomenon are shown in Fig. 139 (page 453). The increase 

 of height is, no doubt, a similar phenomenon to that observed in skeletal muscle, 

 and is probably due to an increase of hydrogen-ion concentration to its optimal 

 value by the lactic acid formed in contraction (see the following section below). 



Action of Ions. The powerful effect of certain inorganic ions has been referred 

 to above (page 143). A few additional facts are of interest in the present 

 connection. The part played by lactic acid suggests that hydrogen ions have an 

 important share in the phenomenon. Mines (1913, 3, p. 221) finds that the 

 optimal concentration of hydrogen ions for the heart is 10~ 72 . If slightly above 

 this, say 10~ 6 - 8 , the beats become slower and weaker, the duration of the 

 electrical change is diminished, while the rate of transmission from auricle to 

 ventricle is decreased (see also Fig. 55, page 187). 



Since increase in frequency of beat results naturally in increase of hydrogen- 

 ion concentration, the above effects may be expected to be met with in such a case. 

 Similarly, with increased rate of stimulation of skeletal muscle, we may expect 

 corresponding results. 



The action of calcium ions is of much importance. We have already described 

 Ringer's work in some detail (pages 207-209). Although calcium is necessary for 

 the occurrence of contractions, it was noticed by Locke and Rosenheim (1907) 

 that a heart at rest, owing to absence of calcium, still continued to consume 

 glucose, and that the electrical change still remained strong. This latter observa- 

 tion was confirmed by Mines (1913, 3, p. 224), and further analysed. It was found 

 that calcium has two effects. It is well known that, as far as its effect on the 

 contractile function is concerned, it cannot be replaced by magnesium. Thus, if 

 we replace a normal fluid by one containing magnesium in place of calcium, the 

 effect on the size of the contractions and on the transmission from auricle to 

 ventricle is the same as if we had merely removed calcium ; but the primary 

 quickening of the rhythm, which is the first effect of a solution devoid of calcium, 

 is absent. So that, as far as this latter effect is concerned, magnesium can replace 

 calcium. It appears that the contractions fail in the absence of calcium because 

 the actual contractile mechanism, on which lactic acid plays, is thrown out of gear 

 in some way. A point of interest, upon which further information is required, is 

 whether there is production of heat, which would be expected to occur when 

 glucose is consumed. It does not seem to me to be a satisfactory explanation to 

 suppose that the contractile substance is in the forms of strands of a calcium salt 

 of some colloidal material, which contracts when acid is formed in contact with it. 

 The various facts referred to on previous pages indicate rather an electrical effect 

 on surface energy, but dogmatic statements are out of place at present. 



Contractile Muscles and Arrest Muscles. In many animals, as we shall see in 

 more detail in Chapter XVIII., the two functions of shortening and of maintenance 

 in the state of shortening arrived at, appear to be assigned to separate muscle 

 fibres of different characteristic properties. In the bivalve molluscs, there is a 

 small quickly contracting muscle which closes the shells ; but the shells are kept 



