[Illustration: Fig. 10.--Cross-section of a small artery in the mesentery. Note that the vessel appears capable of being much widened. The internal elastic lamina is thrown into folds somewhat resembling the convolutions of the brain. Note also that the middle coat of the artery is composed almost entirely of muscle. The enormous number of such vessels in the mesentery and intestines explains the ability of the splanchnic area to accommodate the greater part of the blood in the body. Universal constriction of these vessels would naturally render the intestines anemic. The vasomotor control of these vessels plays an important rôle in the distribution of the blood. Small arteries in the skin and in other organs, possibly the brain, have a similar function. (Microphotograph, highly magnified.)] It is conceivable that in one section of the body the vessels may be markedly contracted, but if there is dilatation in some other part there will be no increased work on the part of the heart, and theoretically, there should be no rise of blood pressure. The vascular system, however, while likened to a system of rubber tubes, must be regarded as a very live system, every subsystem having the property of separate control. For blood tension to be raised all over the body, conditions must favor the generalized contraction of a large area of arterioles. Some authors consider that the so-called viscosity of the blood also is a factor in the causation of increased tension. The usual cause for the high tension is probably the presence in the blood of some poisonous substance. It is held by some authors that the great splanchnic area is capable of holding all the blood in the body and in respect of its liability to arteriosclerosis, it is second only to the aorta and coronary arteries. The enormous area of the skin vessels could probably contain most of the blood. The tone of the vasoconstrictor center controls the distribution of blood throughout the body. The fact that the vessels in the splanchnic area are frequently attacked by sclerotic changes means, as a rule, increase of work for the heart.[1] The resistance offered to the passage of the blood must be great and signifies that, for blood to travel at the same rate that it did before the resistance set in, more power must be expended in its propulsion. In other words, the heart must gradually become accustomed to the changed conditions, and, as a result of increased work, the muscle hypertrophies. (See Fig. 11.)