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

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 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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    the allantoid membrane, which now begins to be developed,— these being the first considerable branches of the iliac arteries which are formed. While this union of the dorsal and abdominal portions of the double aorta takes place, the two vessels arising from the bulb of the heart, of which the aortse formed at first the continua- tion, do not, like these, become united into one trunk, as the observations of M. Serres would lead us to believe. I have al- ready described these two vessels * as the first pair of bran- chial arches, the posterior parts of which form the separate roots of the aorta to be found in the chick on the third and fourth days of incubation ; these roots being also joined at this period by the four other branchial arches which appear succes- sively on each side of the pharynx. These roots of the aorta and branchial arches, we have already remarked, do not become united to one another, but undergo other very remarkable changes, by their partial enlargement or obliteration. Parts of the first branchial arches give rise to the carotid arteries in all vertebrated animals : while the proper trunk of the aorta, or at least its ascending portion and arch, is produced from other branchial vessels, and the roots into which they are joined ; one or more of these serving to form the aorta, according to the class of animals in which the transformation occurs. In Mam- malia, the aorta is formed by the permanence of the fourth branchial arch and the aortic root of the left side; in Birds by that on the right; in the greater number of Reptiles by one on each side; in the tailed Batrachia by three or four arches on each side and by both roots; in Osseous Fishes by four; and in the Sharks, Skates, he. by all the five pairs of branchial ves- sels and the two roots which are to be found in the early stages of development in the foetus f. The diseovery of the double state of the dorsal and abdomi- nal aorta in the very young foetus, made by M. Serres, must, however, be regarded as very interesting, not only as it points out a very singular change, little attended to before it was inves- tigated by this author, taking place in the median arteries, but also as it seems to afford an explanation of some varieties in the • See page C4, &c. and figs. 20, 21, and 30. in the last Plate.