164 PHYSIOLOGY FOR DENTAL STUDENTS. 



tinue to beat for some time if it is properly nourished by perfus- 

 ing blood through it under pressure. The cause of this prop- 

 erty of automatic!!? is still unsettled, and there have been some 

 very interesting discussions and arguments among physiologists 

 concerning it. Some believe that the heart muscle has this prop- 

 erty inherent in itself, and that it originates the impulse which 

 causes the contraction of the heart ; while others think that there 

 are present in the heart-muscle cells of a nervous character whose 

 special function it is to originate the beat. Experimental facts 

 can be found in support of either theory, but the question is still 

 in dispute. Heart muscle differs from other muscle in that each 

 fiber consists of a single cell containing striated protoplasm. It 

 may quite well be that this kind of muscle possesses some char- 

 acteristics usually ascribed to nervous tissue, and that it does 

 originate the stimuli which produce automatic movements. 



The Sequence of the Heart Beat. Inspection of the beating 

 heart of a recently killed turtle or frog shows that the heart beat 

 begins by a contraction in the large veins where they join the 

 auricles. From these vessels the beat spreads, as it were, to the 

 auricles and then to the ventricles, beginning at the base and 

 ending at the apex. It is possible to stop the contraction of the 

 ventricles by drawing a thread tightly around the heart between 

 the auricles and the ventricles. The auricles will continue to 

 beat as before, and the ventricles can be made to beat rhythmical- 

 ly again by artificially stimulating them. In this case, how- 

 ever, they will contract without any reference to the auricular 

 beat. Likewise the base of the large veins, or the sinus venosus 

 as this is known in the amphibian heart, may be separated from 

 the auricles by a tight thread. The auricles now continue to 

 beat, but at a much slower rate, whereas the beat of the sinus 

 is not changed. The tissues of the sinus must possess to a 

 marked degree the power of making individual or automatic 

 movements; they are thus able to control the rate of the heart. 

 For this reason the sinus has been called the cardiac pacemaker. 



The great muscular development of the human heart has 

 caused it to lose some of its primitive characteristics. Neverthe- 

 less, there still exist in the musculature of the heart some strands 



