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(seq. 77)
35 Lect. 3
back, & therefore it must be driven forwards. They are probably in pretty large Veins, & placed irregularly & not in very small ones - If it were not for the Valves, We should be more subject to various Swellings; as they divide a great Length of Column into many smaller ones. The Pressure is less, but this is not the Principal Use of them, as was said before, or else the so great Length of the Vena Cava, would not be without them, but by means of these Valves every lateral Pressure quickens the Circulation, & hurries the Blood towards the Heart - Veins, that lie upon an Artery seem to have a pulsation, which is the only Action of the subjacent Artery - Hence a Surgeon upon opening such a Vein is afraid, that he has wounded an Artery, & this most frequently happens in Fevers, where the Arteries act more strongly Some Veins near the Heart have something like a Pulse, or Swelling & subsiding, which is occasioned by the Action of the Heart, giving a momentary Stop to the Blood, & occasioning an Undulation. In the Veins of the Neck, there is a manifest rising & falling, also in those of the Brain, which is owing to Respiration - The Vein, that answers to an Artery is generally larger, & the Veins have no Pulsation, for the same Reason, that the small ramifying Arteries have none, the Course of the Blood being continual, for the Blood is carried on by
(seq. 79)
36 Lect. 3
two Actions. 1st that of the Heart drives it on, & then the elastic Coat of the Artery acts upon it. The Jugular Veins appear to pulsate sometimes, when there are Polypi in or near the Heart, owing to the Blood's being impeded by them - Not only Veins & Arteries, but all the Vessels of the Body are elastic, & will bear stretching - This our Constitution seemed to require, as our Bodies are liable to so many Changes from Rastings, Overlodgings, loss of Blood &c. By this Elasticity of them we are able to bear a greater or less Quantity of Blood at one Time, than another, the Vessels always dilating or contracting, so as to accomodate themselves to the Quantity of Blood thrown into them, & therefore keeping the System full at all Times. When at any Time the Arteries & Veins have lost their Elasticity, they are supposed to occasion Palpitations of the Heart, sudden Death, Faintings &c., for when the Quantity of Blood is small, & the Vessels so far lost their Elasticity, as not to be able to accomodate themselves to the Quantity of circulating Fluids. The Pulse becomes irregular & intermitting. The Patient is subject to Faintings, Palpitations &c. The Case of Mr. Page is an Instance of this He had been troubled with a Palpitation at the Heart for several Years, & was always much relieved by a full meal,