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SECT.  XXIII.

OF THE CIRCULATORY SYSTEM.

I. The heart and arteries have no antagonist muscles.  Veins absorb the blood, propel it forwards, and distend the heart; contraction of the heart distends the arteries.  Vena portarum. II. Glands which take their fluids from the blood.  With long necks, with short necks. III. Absorbent system. IV. Heat given out from glandular secretions.  Blood changes colour in the lungs and in the glands and capillaries. V. Blood is absorbed by veins, as chyle by lacteal vessels, otherwise they could not join their streams. VI. Two kinds of stimulus, agreeable and disagreeable.  Glandular appetency.  Glands originally possessed sensation.

I. We now step forwards to illustrate some of the phenomena of diseases, and to trace out their most efficacious methods of cure; and shall commence this subject with a short description of the circulatory system.

As the nerves, whose extremities form our various organs of sense and muscles, are all joined, or communicate, by means of the brain, for the convenience perhaps of the distribution of a subtile ethereal fluid for the purpose of motion; so all those vessels of the body, which carry the grosser fluids for the purposes of nutrition, communicate with each other by the heart.

The heart and arteries are hollow muscles, and are therefore indued with power of contraction in consequence of stimulus, like all other muscular fibres; but, as they have no antagonist muscles, the cavities of the vessels, which they form, would remain for ever closed, after they have contracted themselves, unless some extraneous power be applied to again distend them.  This extraneous power in respect to the heart is the current of blood, which is perpetually absorbed by the veins from the various glands and capillaries, and pushed into the heart by a power probably very similar to that, which raises the sap in vegetables in the spring, which, according to Dr. Hale’s experiment on the stump of a vine, exerted a force equal to a column of water above twenty feet high.  This force of the current of blood in the veins is partly produced by their absorbent power, exerted at the beginning of every fine ramification; which may be conceived to be a mouth absorbing blood, as the mouths of the lacteals and lymphatics absorb chyle and lymph.  And partly by their intermitted compression by the pulsations of their generally concomitant arteries; by which the blood is perpetually propelled towards the heart, as the valves in many veins, and the absorbent mouths in them all, will not suffer it to return.

The blood, thus forcibly injected into the chambers of the heart, distends this combination of hollow muscles; till by the stimulus of distention they contract themselves; and, pushing forwards the blood into the arteries, exert sufficient force to overcome in less than a second of time the vis inertiae, and perhaps some elasticity, of the very extensive ramifications of the two great systems of the aortal and pulmonary arteries.  The power necessary to do this in so short a time must be considerable, and has been variously estimated by different physiologists.