466 



SCIENTIFIC NEWS. 



[Nov. 2, iS 



THE BEATING OF THE HEART. 



T7EW of the marvellous phenomena of our own 

 physical life excite our curiosity so early or so 

 keenly as the throbbing of the heart in the breast. The 

 child speculates as to the cause and object of this rapid 

 and incessant pulsation, and only gives up his inquiry 

 because the problem is too hard. The expert physio- 

 logist spends a working life in getting to understand 

 fully and experimentally little bits of the driving engine 

 of the blood-circulation. Toil as he may, the explana- 

 tion never grows perfectly simple. Special facts, 

 gradually seen to be important, but at first sight 

 irrelevant or superfluous, break up that easy and 

 luminous theory which is still hoped for, never found. 

 Perhaps it is not untrue to say that the more the heart 

 is studied, the harder it is to understand. 



We shall only succeed in giving the reader a clear 

 view of the cardinal facts by leaving out altogether a 

 host of recorded details which the working physiologist 

 is bound to keep steadily in view. Let us attempt to 

 answer but two questions — 



1. How is it that the heart never becomes fatigued ? 



2. How is its rhythmical beat kept up? 



The simplest answer to question I is that the heart 

 does become fatigued. Indeed, it becomes fatigued very 

 quickly, but recovers itself after a very short rest. We 

 have heard of men who undertake for a wager to walk 

 1,000 miles in 1,000 hours. The heart undertakes to 

 contract, say, 5,000 times in an hour. It becomes 

 fatigued, and rests after each contraction, and this inter- 

 val of fatigue is in itself enough to occasion a certain 

 kind of rhythmical action, of which examples could be 

 quoted. The motive power is put forth in response to 

 stimulus, and is exhausted in a moment'. A short spell 

 of rest enables the stimulus to provoke a fresh contrac- 

 tion. This leads us to ask where the motive power is 

 stored up. The answer is furnished by experiment 

 upon a fresh-killed frog. Cut out the heart; it still con- 

 tinues to beat. Cut it across just |below the line which 

 marks the junction of the ventricle with the two auricles; 

 the upper half continues to beat, but the lower half is 

 paralysed. Divide the upper half into two ; each 

 quarter continues to beat. There must therefore be 

 motive centres within the heart sufficient to keep up 

 pulsation. Indeed, by a somewhat difficult and minute 

 dissection these centres can be found. They are minute 

 ganglia, or groups of nerve-cells. 



Given ganglia in communication with the muscular 

 fibres of the heart, a rhythmical contraction at once be- 

 comes possible. Some stimulus, e.g., the pressure of 

 blood in the auricles, excites the ganglia. These stimu- 

 late the muscles, contraction follows, and the impulse is 

 spent. The nervous force has been discharged, and 

 requires time for recovery. The muscular force has 

 also been discharged, and requires time for recovery. 

 It is probably in this very simple way that detached 

 organs which contain ganglia are occasionally seen to 

 beat. A frog's tongue, the foot of a fresh-water mussel, 

 or a small strip of the bell of a jelly fish may pulsate 

 after removal from the body, provided that it contains a 

 ganglion in communication with muscular fibres. It is 

 possible, though not proved, that the muscular fibres of 

 the intestine, and the muscular fibres of the heart in 

 various animals of low grade, may keep up their peri- 

 staltic action in the same way. 



But in all hearts which have been carefully and expe- 



rimentally studied a more complicated piece of machi- 

 nery is added. This takes the form of an inhibiting 

 nerve, which checks the ganglion before its force is 

 wholly spent. Inhibition is just as effective as fatigue 

 in keeping up a rhythmical movement. The sequence 

 now becomes — (1) stimulation of the cardiac ganglia; 

 (2) the overpowering by accumulated stimulus of the 

 inhibiting force ; (3) discharge ; (4) renewed inhibi- 

 tion. 



In higher animals the inhibiting nerve is a long nerve, 

 called the pneumogastric, which traverses the neck and 

 originates in the medulla oblongata, at the back of the 

 brain. Irritation of the pneumogastric by an inter- 

 rupted electric current, pressure, or chemical stimula- 

 tion may be made to produce a long-continued stoppage 

 of the heart Czermak was able to stop his heart at 

 pleasure by pressing this nerve against a small bony 

 tumour in his neck. The regulation of the rate and 

 force of the heart's beat is by means of the pneumo- 

 gastric transferred to the brain, and brought into rela- 

 tion with what is going on in distant parts of the body. 

 If the body of a frog is laid open, and a light but smart 

 blow is given to the intestine, the heart will stand still. 

 But if the medulla or both the pneumogastric nerves 

 have been previously destroyed, no such effect follows. 

 It was the impulse transmitted to the medulla from the 

 intestine which stimulated the inhibitory nerve, but 

 when the nervous path is broken the heart is alto- 

 gether out of relation with the intestine. 



Czermak (Physiologische Vortrage, I.) has devised a 

 model which enables the student to gain a clear idea of 

 the combined effect of stimulus and inhibition in pro- 

 ducing rhythmical motion. A stream of water, regu- 

 lated by a tap, descends upon what he calls a boat. This 

 boat is poised upon an axis. It has a flat base, and a 

 partition across its middle, at right angles to the two 

 triangular sides. At starting the boat is tilted up, one 

 of its two compartments being turned upwards, while 

 the other rests upon an adjustable support. The stream 

 flows into the upper compartment, and fills it to the 

 point at which it counterbalances the other. Then the 

 boat capsizes, and empties itself. The other compart- 

 ment now begins to fill, and so en. It is a pretty enough 

 sight to see the boat oscillating by the action of a constant 

 stream of water. 



In this model we have a cause of movement — viz., the 

 stream of water, and a cause of stoppage — viz., the 

 support beneath, which checks the fall of the boat. The 

 joint effect of the motive and restraining forces is a 

 rhythmical action. If the fall is increased by lowering 

 the support beneath the boat, the oscillation becomes 

 slower and more forcible. If the support is raised the 

 oscillation becomes quicker and weaker. Increase or 

 diminution of the stream affects the rate, but not the 

 force, which is entirely determined by the amount of 

 fall. The model gives us, therefore, four possible com- 

 binations of rate and force. The oscillations may be 

 (1) strong — quick, (2) strong — slow, (3) weak — quick, (4) 

 weak — slow, and the very same combinations are met 

 with in the pulse. ■ It is not too much to say that Czer- 

 mak's model is a trustworthy guide to the cause of each 

 variation. 



Into other details of the movements of the heart we 

 shall not enter at present. The student or teacher who 

 will make Czermak's boat, and master its analogy with 

 the pulsating heart, has won a solid foundation for fur- 

 ther observation and reflection. 



