A Practical Physiology: A Text-Book for Higher Schools

by Albert F. Blaisdell

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.

The effect of this power of the nervous system is to give it a certain control over the circulation in particular parts. Thus, though the force of the heart and the general average blood-pressure remain the same, the state of the circulation may be very different in different parts of the body. The importance of this local control over the circulation is of the utmost significance. Thus an organ at work needs to be more richly supplied with blood than when at rest. For example, when the salivary glands need to secrete saliva, and the stomach to pour out gastric juice, the arteries that supply these organs are dilated, and so the parts are flushed with an extra supply of blood, and thus are aroused to greater activity.

Again, the ordinary supply of blood to a part may be lessened, so that the organ is reduced to a state of inactivity, as occurs in the case of the brain during sleep. We have in the act of blushing a visible example of sudden enlargement of the smaller arteries of the face and neck, called forth by some mental emotion which acts on the vaso-motor center and diminishes its activity. The reverse condition occurs in the act of turning pale. Then the result of the mental emotion is to cause the vaso-motor nerves to exercise a more powerful control over the capillaries, thereby closing them, and thus shutting off the flow of blood.

Experiment 91. Hold up the ear of a white rabbit against the light while the animal is kept quiet and not alarmed. The red central artery can be seen coursing along the translucent organ, giving off branches which by subdivision become too small to be separately visible, and the whole ear has a pink color and is warm from the abundant blood flowing through it. Attentive observation will show also that the caliber of the main artery is not constant; at somewhat irregular periods of a minute or more it dilates and contracts a little.