On the development of the vascular system in the foetus of vertebrated animals. Part II. / [Allen Thomson].

  • Thomson, Allen, 1809-1884
Date:
1831]
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Licence: Public Domain Mark

Credit: On the development of the vascular system in the foetus of vertebrated animals. Part II. / [Allen Thomson]. Source: Wellcome Collection.

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    Vertehrated Animals. Part II. By Allen Thomson, M. D late President of the Royal Medical Society.—Continued. From the Edinburgh New Philosophical Journal for January 1831. Having in the former part of this essay considered the mode of formation of the Heart in the different orders of vertehrated animals, I shall now give some account of that of the other parts of the vascular system ; viz. of the Bloodvessels of the body. There seem to be two modes principally in which bloodvessels are developed ; the one, by isolated points and vessels, has been already alluded to, in the account given of the commencement of the circulation, as it occurs on the vascular area of the yolk ; the other, taking place after the commencement of the circula¬ tion, by the prolongation of loops or folds from vessels already formed, is most easily seen on the transparent parts of the Batrachian reptiles. The sac of the yolk, or covering which the yolk receives from the layers of the germinal membrane, is the part on which, in all vertehrated animals, the blood and vessels appear to origi¬ nate, and it is the only part in which, in healthy animals, the - formation of bloodvessels has been observed to take place inde¬ pendently of the heart or general circulation. During the de¬ velopment of the vascular area (to the detail of which it is now unnecessary to recur), no difference has as yet been observed between the mode of the formation of arteries and veins. The blood appears to circulate sooner in the veins than in the arteries of the area, but, in the early stages of development, these vessels are to be distinguished from one another only by their distri¬ bution, and the direction of the currents of blood in them. About the fourth or fifth day of incubation, the coats of the arteries begin to appear thicker than those of the veins, and very soon the external appearance of these vessels affords a character sufficientlv distinctive. As far as has been ascer- v tained, there does not appear to be any immediate connexion between the formation of vessels in the area, and that of the heart itself: these processes seem, for a time at first, to go on simultaneously, but independently of one another; and, in- D