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 nerves which thus regulate the calibre of the small arteries by acting on their muscular coats are called vaso-motor nerves; and through them the nervous system is able to exert a local control over the circulation in any part or organ, the importance of which is very great. Thus, when an organ becomes active, it is of advantage that it should be more richly supplied with blood than when it is at rest. Accordingly we find that when a muscle contracts, or when a salivary gland secretes saliva, or when the stomach is preparing to digest food, in each case the small arteries of the muscle, salivary gland or stomach, dilate and so flush the part with blood. The organ in fact blushes; and this inner unseen blushing is, like the ordinary blushing described above, brought about by vaso-motor nerves. We shall see later on that the temperature of the body is largely regulated by the supply of blood sent to the skin to be cooled, and this supply is in turn regulated by the vaso-motor nervous system. Indeed everywhere all over the body, the nervous system by its vaso-motor nerves is continually supervising and regulating the supply of blood, sending now more now less blood, to this or that part; and many diseases, such as those when exposure to cold causes congestion or inflammation, are due to, or at least associated with, a disorder or failure of this vaso-motor activity. 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 every one 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. There are Now the heart is well supplied with nerves. many small ganglia, or masses of nerve cells lodged in the substance of the heart, more especially in the auricles, and nerves spread from these ganglia over the walls, both of the auricles and ventricles. Moreover, several nerves reach the heart from the outside. Of these the most important perhaps are branches of a remarkable nerve which starts from the brain, and supplies not only the heart, but the lungs, alimentary canal, and other parts, and which is called the pneumogastric, or from its wandering course, the vagus. Other nerves reaching the heart seem to come from the sympathetic, but probably many of these may be traced back through the sympathetic to the spinal cord. There is every reason to believe that the regular rhythmical succession of the ordinary contractions of the heart depends in some way upon the ganglia lodged in its substance. At any rate, it is certain that these movements do not depend on any nerves outside the heart, since they go on even when the heart is removed from the body. On the other hand the influence which arrests the heart's action, as in fainting, comes to the heart from without, and is carried to the heart 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 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. 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 circulation. 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. 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. When a person faints from a sudden emotion, a similar influence, started in the brain, descends along the pneumogastric, and similarly stops for a while the beating of the heart. The exact manner in which palpitation is brought about does not seem so clear; in such cases an influence of some kind probably reaches the heart along nerves which for a part of their course run along with the sympathetic nerves ; but this subject requires further investigation. The 28. The evidence that the blood circulates in man, although perfectly conclusive, is almost all indirect. 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 comes from the other end. When, however, we tie a vein |