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the capillaries of the heart is carried back by the coronary vein, not to either vena cava, but to the right auricle. The opening of the coronary vein is protected by a valve, so as to prevent the right auricle from driving the venous blood which it contains back into the vessels of the heart.
22. Thus, the shortest possible course which any particle of the blcod can take in order to pass from one side of the heart to the other, is to leave the aorta by one of the coronary arteries, and return to the right auricle by the coronary vein. And in order to pass through the greatest possible number of capillaries and return to the point from which it started, a particle of blood must leave the heart by the aorta and traverse the arteries which supply the alimentary canal, spleen, and pancreas. It then enters, Istly, the capillaries of these organs; 2ndly, the capillaries of the liver; and, 3rdly, after passing through the right side of the heart, the capillaries of the lungs, from which it returns to the left side and eventually to the aorta.
Furthermore, from what has been said respecting the lymphatic system, it follows that any particle of matter which enters a lacteal of the intestine, will reach the right auricle by the superior cava, after passing through the lymph capillaries and channels of sundry lymphatic glands; while anything which enters the adjacent blood capillary in the wall of the intestine will reach the right auricle by the inferior cava, after passing through the blood capillaries of the liver.
23. It has been shown above (§ 2) that the small arteries may be directly affected by the nervous system, which controls the state of contraction of their muscular walls, and so regulates their calibre. The effect of this power of the nervous system is to give it a certain control over the circulation in particular spots, and to produce such a state of affairs that, although the force of the heart and the general condition of the vessels remain the same, the state of the circulation may be very different in different localities.
Blushing is a purely local modification of the circulation of this kind, and it will be instructive to consider how a blush is brought about. An emotion-sometimes pleasurable, sometimes painful-takes possession of the
mind : thereupon a hot flush is felt, the skin grows red, and according to the intensity of the emotion these changes are confined to the cheeks only, or extend to the roots of the hair," or all over.”
What is the cause of these changes ? The blood is a red and a hot fluid ; the skin reddens and grows hot, because its vessels contain an increased quantity of this red and hot fluid ; and its vessels contain more, because the small arteries suddenly dilate, the natural moderate contraction of their muscles being superseded by a state of relaxation. In other words, the action of the nerves which cause this muscular contraction is suspended.
On the other hand, in many people, extreme 'terror causes the skin to grow cold, and the face to appear pale and pinched. Under these circumstances, in fact, the supply of blood to the skin is greatly diminished, in consequence of an excessive stimulation of the nerves of the small arteries, which causes them to contract and so to cut off the supply of blood more or less completely.
24. That this is the real state of the case may be proved experimentally upon rabbits. These animals may be made to blush artificially. If, in a rabbit, the sympathetic nerve which sends branches to the vessels of the head is cut, the ear of the rabbit, which is covered by so delicate an integument that the changes in its vessels can be readily perceived, at once blushes. That is to say, the vessels dilate, fill with blood, and the ear becomes red and hot. The reason of this is, that when the sympathetic is cut, the nervous stimulus which is ordinarily sent along its branches is interrupted, and the muscles of the small vessels, which were slightly contracted, become altogether relaxed.
And now it is quite possible to produce pallor and cold in the rabbit's ear. To do this it is only necessary to irritate the cut end of the sympathetic which remains connected with the vessels. The nerve then becomes excited, so that the muscular fibres of the vessels are thrown into a violent state of contraction, which diminishes their calibre so much that the blood can hardly make its way through them. Consequently, the ear becomes pale and cold.
25. The practical importance of this local control exerted by the nervous system is immense.
When exposure to cold gives a man catarrh, or inflammation of the lungs, or diarrhea, or some still more serious affection of the abdominal viscera, the disease is brought about through the nervous system. The impression made by the cold on the skin is conveyed to the nervous centres, and so influences the vaso-motor nerves (as the nerves which govern the walls of the vessels are called) of the organ affected as to cause their partial paralysis, and produce that state of congestion (or undue distension of the vessels) which so commonly ends in inflammation. (See Lesson XI. § 15.)
26. Is the heart, in like manner, under the control of the central nervous system?
As we all know, it is not under the direct influence of the will, but everyone is no less familiar with the fact that the actions of the heart are wonderfully affected by all forms of emotion. Men and women often faint, and have sometimes been killed by sudden and violent joy or sorrow ; and when they faint or die in this way, they do so because the perturbation of the brain gives rise to a something which arrests the heart as dead as you stop a stop-watch with a spring. On the other hand, other emotions cause that extreme rapidity and violence of action which we call palpitation.
Now there are three sets of nerves in the heart : one set are supplied by ganglia, or masses of nerve-cells, in its substance; another set come from the sympathetic nerve; a third set are branches of a remarkable nerve, which proceeds straight from the brain, and is called the pneumogastric nerve. There is every reason to believe that the regular rhythmical succession of the ordinary contractions of the heart depends upon the ganglia lodged in its substance. At any rate, it is certain that these movements depend neither on the sympathetic, nor on the pneumogastric, since they go on as well when the heart is removed from the body.
In the next place, there is much reason to believe that the influence which increases the rapidity of the heart's action is exerted through the sympathetic.
And lastly, it is quite certain that th einfluence which arrests the heart's action is supplied by the pneumogastric. This may be demonstrated in animals, such as frogs, with great ease.
27. If a frog be pithed, or its brain destroyed, so as to
Fig. 15.-Portion of the web of a frog's foot seen under a low magnifying power, the blood vessels only being represented except in the corner of the field, where in the portion marked off the pigment spots are also drawn. a. small arteries; v. small veins': the minute tubes joining the arteries of the
veins are the capillaries. The arrows denote the direction of the circu'ation. The larger artery running straight up in the middle line breaks up into capillaries at points higher up than can be shown in the drawing.
obliterate all sensibility, the animal will continue to live, and its circulation will go on perfectly well for an indefinite period. The body may be laid open without causing pain or other disturbance, and then the heart will be observed beating with great regularity. It is possible to make the heart move a long index backwards and forwards; and if frog and index are covered with a glass shade, the air under which is kept moist, the index will vibrate with great steadiness for a couple of days.
It is easy to adjust to the frog thus prepared a contrivance by which electrical shocks may be sent through the pneumogastric nerves, so as to irritate them. The moment this is done the index stops dead, and the heart will be found quiescent, with relaxed and distended walls. After a little time the influence of the pneumogastric passes off, the heart recommences its work as vigorously as before, and the index vibrates through the same arc as formerly. With careful management, this experiment may be repeated very many times ; and after every arrest by the irritation of the pneumogastric, the heart resumes its work.
28. The evidence that the blood circulates in man, although perfectly conclusive, is almost all indirect. The most important points in the evidence are as follows :
In the first place, the disposition and structure of the organs of circulation, and more especially the arrangement of the various valves, will not, as was shown by Harvey, permit the blood to flow in any other direction than in the one described above. Moreover, we can easily with a syringe inject a fluid from the vena cava, for instance, through the right side of the heart, the lungs, the left side of the heart, the arteries, and capillaries, back to the vena cava; but not the other way. In the second place, we know that in the living body the blood is continually flowing in the arteries towards the capillaries, because when an artery is tied, in a living body, it swells up and pulsates on the side of the ligature nearest the heart, whereas on the other side it becomes empty, and the tissues supplied by the artery become pale from the want of a supply of blood to their capillaries. And when we cut an artery the blood is pumped out in jerks from the cut end nearest the heart, whereas little or no blood