I, and indeed others, have sometimes found valves in the emulgent veins, and in those the mesentery, the edges which were directed towards the vena and vena portæ.
Let added that there are no valves in the arteries, and that dogs, oxen, best online essay writers etc.
have invariably valves at the divisions their crural veins, in the veins that meet towards the top the as sacrum, and in those branches which come from the haunches, in which no such effect gravity from the erect position was apprehended. Neither are there valves in the jugular veins for the purpose guarding against apoplexy, as some have said because in sleep the head more apt influenced the contents find someone to write my college paper the carotid arteries. Neither are the valves present, in order that the blood may retained in the divarications or smaller trunks and minuter branches, and not suffered flow entirely into the more open and capacious channels for they occur where there are no divari cations although must owned that they are most frequent at the points where branchès join. Neither they exist for the purpose rendering the current blood more slow from the centre the body for seems likely that the blood would disposed flow with sufficient slowness its own accord, as would have pass from larger into continually smaller vessels, being separated from the mass and fountain head, and attaining from warmer into But the valves are solely made and instituted lest the blood should pass from the greater into the lesser veins, and either rupture them or cause them become varicose lest, instead advancing from the extreme the central parts the body, the blood should rather proceed along the veins from the centre the extremities but the delicate valves, while they readily open in the right direction, entirely prevent all such contrary motion, being situated and arranged, that if anything escapes, or less perfectly obstructed the cornua the one above, the fluid passing, as were, the chinks between the cornua, immediately received the convexity the one beneath, which placed transversely with reference the former, and effectually hindered from getting any farther. And this I have frequently experienced in dissections the veins if I attempted pass a probe from the trunk the veins into one the smaller branches, whatever care I took I found impossible introduce far any way, reason the valves whilst, the contrary, was most easy push along in the opposite direction, from without inwa or from the branches towards the trunks and roots.
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In many places two valves are placed and fitted, that when raised they come exactly together in the middle the vein, and are there united the contact their l argins and accurate the adaptation, that neither the eye nor any other means examination, can the slightest chink along the line contact perceived. But if the probe now introduced from the extreme writing thesis towards the more central parts, the valves, like the floodgates a river, give way, and are most readily pushed aside.
The effect this arrangement plainly prevent all motion the blood from the heart and vena cava,vhether upwards towards the head, or downwards towards the feet, or either side towards the arms, not a drop can pass all motion the blood, beginning in the larger and tending towards the smaller veins, opposed and resisted them whilst the motion that pro.
ceeds from the lesser end in the larger branches favoured, at all events, a free and open passage left for But that this truth may made the more apparent, let an arm tied above the elbow as if for phlebotomy A, A, I. At intervals in the course the veins, especially in labouring people and those whose veins are large, certain knots or elevations B, D, E, F will perceived, and this not only at the places where a branch received E, F, but also where none enters D these knots or risings are all formed valves, which thus show themselves externally. And now if you press the blood from the space above one the valves, from H, and keep the point a finger upon the vein inferiorly, you will see no influx blood from above the portion the vein between the point the finger and the valve will obliterated yet will the vessel continue sufficiently distended above the valve, G. The blood being thus pressed out and the vein emptied, if you now apply a finger the other hand upon the distended part the vein above the valve, and press downwards, you will find that you cannot force the blood through or beyond the valve but the greater effort you use, you will only see the portion vein that between the finger and the dissertation abstracts international valve become more distended, that portion the vein which below the valve remaining all the while empty It would therefore appear that the function the valves in the veins the same as that the three sigmoid valves which find at the commencement the aorta and pulmonary artery, prevent all reflux the blood that passing over them. NoTE.
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Woodcuts the veins the arm which these letters and figures refer appear here in the original.
Farther, the arm being bound as before, and the veins looking full and distended, if you press at one part in the course a vein with the point a finger L, and then with another finger streak the blood upwards beyond the next valve N, you will per ceive that this portion the vein continues empty N, and that the blood cannot retrograde, precisely as have already seen the case but the finger first applied, being removed, immediately the vein filled from below, and the arm becomes as appears at D That the blood in the veins therefore proceeds from inferior or more remote parts, and towards the heart, moving in these vessels in this and not in the contrary direction, appears most obviously. And although in some places the valves, not acting with such perfect accuracy, or where there but a single valve, not seem totally prevent the passage the blood from the centre, still the greater number them plainly and then, vhere things appear contrived more negligently, this compensated either the more frequent occurrence or more perfect action the succeeding valves, or in some other the veins in short, as they are the free and open conduits the blood returning the heart, are they effectually prevented from serving as its channels distribution from the heart. But this other circumstance has noted The arm being bound, and the veins made turgid, and the valves prominent, as before, apply the thumb or finger over a vein in the situation one the valves in such a way as compress and prevent any blood from passing vards from the hand then, with a finger the other hand, streak the blood in the vein upwards till has passed the next valve above, the vessel now remains empty but the finger at being removed for an instant, the vein immediately filled from below apply the finger again, and having in the same manner streaked the blood upwards, again remove the finger below, and again the vessel becomes distended as before and this repeat, say a thousand times, in a short space time, And now compute the quantity blood which you have thus pressed beyond the valve, and then multiplying the assumed quantity one thousand, you will find that much blood has passed through a certain portion the vessel and I now believe that you will find yourself convinced the circulation the blood, and its rapid motion. But if in this experiment you say that a violence done nature, I not doubt but that, if you proceed in the same way, only taking as great a length vein as possible, and merely remark with what rapidity the blood flows upwards, and fills the vessel from below, you will come the same conclusion. AND now I may allowed give in brief view the circulation the blood, and propose for general adoption. Since all things, both argument and ocular demonstration, sho that the blood passes through the lungs, and heart the force the ventricles, and sent for distribution all parts the body, vhere makes its way into the veins and porosities the flesh, and then flows the veins from the circumference every side the centre, from the lesser the greater veins, and them finally discharged into the vena cava and right auricle the heart, and this in such a quantity or in such a flux and reflux thither the arteries, hither the veins, as cannot possibly supplied the ingesta, and much greater than can required for mere purposes nutrition absolutely necessary conclude that the blood in the animal body impelled in a circle, and in a state ceaseless motion that this the act or function which the heart performs means its buying research papers pulse and that the sole and only end the motion and contraction the heart. IT will not foreign the subject college essay writing consultant if I here show further, from certain familiar reasonings, that the circulation matter both convenience and necessity. In the first place, since death a corruption which takes place through deficiency heat, and since all living things are,varm, all dying things cold, there must a particular seat and fountain, a kind home and hearth, where the cherisher nature, the original the native fire, stored and preserved from which heat and life are dispensed all parts as from a fountain head from which sustenance may derived and upon which concoction and nutrition, and all vegetative energy may depend. Now, that the heart this place, that the heart the principle life, and that all passes in the manner just mentioned, The blood, therefore, required have motion, and indeed such a motion that should return again the heart for sent the external parts the body far from its fountain, as Aristotle says, and without motion, would become congealed.