Collateral with the minute ramifications of the veins and arteries, over the whole system, are the _lymphatic vessels_, which separate from the blood as it passes along, a transparent lymphatic humour, adapted for the different parts, which they are connected with.[6] These vessels are materially different, from either arteries and veins in their structure, and justly to be compared to a continuation of little cells; so constructed, as to transport the humours, without admitting a return.
An opinion has lately been broached, that the lymphatics are all absorbing vessels, of the same nature with the lacteals, and that they all join to empty themselves in the subclavian vein to the common mass of the blood; but this to me, seems too general, and contradicts experience.
From hence we see plainly how these three organs, the _brains_, the _lungs_, and the _heart_, are concerned, and mutually assist each other in the actions, and circulation of the fluid of life; nothing could move without the sensation of the nerves; these could not exist without the blood, from which they receive their support; the blood could not circulate without the heart, arteries, and veins; and in them it could not move with a necessary freedom, except its being supplied with a sufficiency of air from the lungs: So that evidently all three are so interested in this grand movement, that the stopping of the one, must needs be the stopping of the whole.
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In the preceding we have briefly considered the state of the fluid of life, and mechanical instruments of their motion in the human machine. In this we will consider how the whole is supported, and the vital lamp, from time to time, becomes supplied.
Under this consideration we will examine the canal of food, and the various changes the morsel undergoes from the time we take it in our mouths, till the nutriment is reduced to blood, and the remaining dross is expelled the body as useless.
The animal functions towards _chylifaction_ and _nutrition_, are _mastication_, or chewing the _food_; _deglutition_, or swallowing; _digestion_; _chylifaction_; _nutrition_; and the _excretion_ of the _fæces_.
In order therefore to understand this clearly, it becomes necessary to describe the organs which nature has formed for the requisite performances thereof.