On the development of the vascular system in the foetus of vertebrated animals. Part 2 / by Allen Thomson.

Date:
[1831]
Catalogue details

Licence: Public Domain Mark

Credit: On the development of the vascular system in the foetus of vertebrated animals. Part 2 / by Allen Thomson. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    sels join their respective ventricles, they appear to cross, or to be twisted round one another. As development proceeds, the root of the aorta is more com- pletely separated from that of the pulmonary artery, and a re- markable change, at the same time, takes place in the direction of the blood through the vessels into which they lead. The blood, propelled by the contraction of the left ventricle through the aortic root (R), instead of going as before into all the vas- cular branches rising on the fore part of the neck, now passes only into the fourth vascular arch on the right side, and the two arteriae innominate which arise from it (t, u, r, s) ; while the blood from the right ventricle is sent through the pulmonary root (P) exclusively into the fourth arch on the left side, and the two fifth or posterior arches (p. p) *. The fourth arch now becomes gradually larger, and becomes the freest mode of communication between the ascending and descending aorta); it forms, in fact, the proper arch of the aorta. In the mean time, the fifth arch on the left side becomes less, gradually carries a smaller quantity of blood, and soon after the separation of the aortic and pulmonary roots is entirely obliterated. Three arches only, therefore, now remain, viz. the fourth on the right side or the trunk of the aorta itself, its corresponding arch on the left side, and the fifth on the right, the two last of which soon after give rise to the pulmonary arteries. On the fifth and sixth days, the parietes of the pharyngeal or branchial cavity of the chick also undergo a transformation. Ac- cording to Ilathke, the part intervening between the mouth and the first pair of branchial apertures becomes thicker and firmer, and is divided by a transverse groove into two portions (Figs. £4 and 25.) The anterior of these (s), bulging out at the sides, forms by its farther development the inferior maxilla ; the pos- terior (a), smooth and projecting less, gives rise to a pendulous fold which overlaps the first branchial slit, and which this au- thor compares to the operculum of fishes f. When the branchi- • Baer explains this change by supposing that, when the roots of the pul- monary artery and aorta are separated, the blood acquires a new direction and is thus driven into their respective vessels. t From the drawings which Ilathke has given of the foetal fish, there can be no doubt of the correctness of this comparison ; but he seems to have omitted to observe, that, before the formation of the opercular body in the foetus of the chick, the most anterior branchial aperture is closed.