1 88 Jourjial of Comparative Neurology and Psychology. 



cosis a systolic refractory period in the sense of diminished excitabihty is in evidence 

 as long as the heart retains excitability and contractility. (2) Nerve tissue prob- 

 ably dies sooner than muscular tissue when circulation and nutrition are stopped. 

 If the refractory state depends on the nervous tissue in the heart, the excised or 

 dying heart ought to exhibit a condition of diminished or abolished refractory 

 state in the later stages while it still retains some excitability and contractility. 

 But even in the last stages of dying the ventricular tissue or tissues retain the prop- 

 erty of systolic refractory state in the sense of diminished excitability. (3) The 

 sodium chloride rhythm of the ventricular apex is probably idio-muscular. In case 

 the refractory period is a property of the nervous tissue alone we might expect a 

 diminution or abolition of the systolic refractory state in this rhythm. But fresh 

 ventricular strips exhibit practically as marked refractory state in the sodium 

 chloride rhythm as in the normal rhythm. 



It is therefore evident that the question whether heart muscle when isolated from 

 the intrinsic nervous tissues exhibits the property of refractory state to greater degree 

 than skeletal and smooth muscle is still an open one, since the facts hearing on the 

 question can he interpreted either way. In Limulus the heart ganglion exhibits the 

 typical refractory period of heart tissue, and as this is a characteristic of at least 

 many ganglion cells in the central nervous system of vertebrates, it is probable 

 that the ganglion cells in the vertebrate heart possess a systolic refractory state 

 similar to that of the Limulus heart ganglion. 



II. The Degree of Refractory State in the Heart of Different Animals, and in 

 the Different Parts of the Heart of the Same Animal. I. The ventricles of higher 

 vertebrates do not exhibit the same degree of refractory state. A strong induction 

 shock sent through the ventricle at the beginning of systole produces a super- 

 maximal beat in the frog, toad and salamander, but not in the tortoise. In the 

 case of the latter the strong induction shock diminishes the amplitude of the beat. 

 But inasmuch as the inhibition of a phenomenon is just as much evidence of irrit- 

 ability as the augmentation of it, it is obvious that even the tortoise ventricle is 

 excitable at the beginning of systole. The refractory state in the heart is therefore 

 a condition of diminished excitability and not a state of absolute inexcitability. 



2. The degree of refractory state is not necessarily the same in the different 

 parts of the heart of the same animal. In one tortoise (Cistudo) supermaximal 

 beats are readily produced in the sinus venosus by stimulation at the beginning 

 of systole, while the ventricle responds to the same stimulation with a diminution 

 of the beat. 



3. There is probably no causal connection between the property of automatism 

 and the property of refractory state, for the following reasons: (i) The property 

 of refractory state is exhibited by tissues that do not have the property of auto- 

 matism under normal conditions — the apex of the frog and tortoise ventricle, 

 nerve centers or ganglion cells in the central nervous system, the mammalian intes- 

 tines (Magnus), the nerve plexus and heart muscle of Limulus. (2) Some tissues 

 that are normally automatic do not exhibit an absolutely refractory state, but only 

 a condition of greatly diminished excitability — the hearts of invertebrates, the 

 hearts of many vertebrates. (3) In the same heart the parts possessing the great- 

 est degree of automatism may exhibit a less degree of refractory state than the 

 part of the heart not automatic. 



