A, superficial fibers common to both ventricles;
  B, fibers of the left ventricle;
  C, deep fibers passing upwards toward the base of the heart;
  D, fibers penetrating the left ventricle
]

The sounds of the heart are modified or masked by blowing “murmurs” when the cardiac orifices or valves are roughened, dilated, or otherwise affected as the result of disease.  Hence these new sounds may often afford indications of the greatest importance to physicians in the diagnosis of heart-disease.

194.  The Nervous Control of the Heart.  The regular, rhythmic movement of the heart is maintained by the action of certain nerves.  In various places in the substance of the heart are masses of nerve matter, called ganglia.  From these ganglia there proceed, at regular intervals, discharges of nerve energy, some of which excite movement, while others seem to restrain it.  The heart would quickly become exhausted if the exciting ganglia had it all their own way, while it would stand still if the restraining ganglia had full sway.  The influence of one, however, modifies the other, and the result is a moderate and regular activity of the heart.

The heart is also subject to other nerve influences, but from outside of itself.  Two nerves are connected with the heart, the pneumogastric and the sympathetic (secs. 271 and 265).  The former appears to be connected with the restraining ganglia; the latter with the exciting ganglia.  Thus, if a person were the subject of some emotion which caused fainting, the explanation would be that the impression had been conveyed to the brain, and from the brain to the heart by the pneumogastric nerves.  The result would be that the heart for an instant ceases to beat.  Death would be the result if the nerve influence were so great as to restrain the movements of the heart for any appreciable time.

Again, if the person were the subject of some emotion by which the heart were beating faster than usual, it would mean that there was sent from the brain to the heart by the sympathetic nerves the impression which stimulated it to increased activity.

195.  The Nervous Control of the Blood-vessels.  The tone and caliber of the blood-vessels are controlled by certain vaso-motor nerves, which are distributed among the muscular fibers of the walls.  These nerves are governed from a center in the medulla oblongata, a part of the brain (sec. 270).  If the nerves are stimulated more than usual, the muscular walls contract, and the quantity of the blood flowing through them and the supply to the part are diminished.  Again, if the stimulus is less than usual, the vessels dilate, and the supply to the part is increased.

Now the vaso-motor center may be excited to increased activity by influences reaching it from various parts of the body, or even from the brain itself.  As a result, the nerves are stimulated, and the vessels contract.  Again, the normal influence of the vaso-motor center may be suspended for a time by what is known as the inhibitory or restraining effect.  The result is that the tone of the blood-vessels becomes diminished, and their channels widen.