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

  • 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 II. / [Allen Thomson]. Source: Wellcome Collection.

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    pharynx. At first, this vessel divides itself only into two branches situated immediately behind the mouth, which, passing round the pharynx, reunite with one another on its upper part, below the vertebral column, to form the descending aorta. Shortly afterwards, a farther subdivision of the aorta takes place, by which four new vascular branches (/;?) are formed on each side of the pharynx, behind the one which appeared first, and leading from the ascending to the descending trunks of the aorta. These vessels, the primitive branchial arteries, are at first nearly of a uniform diameter, and do not divide into any branches in their course round the pharynx. Each artery runs along one of the branchial plates or hoops, (Fig. 2, I). As development proceeds, four transverse clefts, the com¬ mencing branchial apertures, appear between the branchial hoops on the lower and lateral parts of the pharynx (Figs. 2 and 3, o). The four posterior hoops become stronger, more car¬ tilaginous, and project farther from the side of the oesophagus, and, at the same time, little leaflets or tubercles, the rudiments of those which afterwards form the comb-like fringe of the gill, begin to be formed on their external sides. Each of these leaf¬ lets, soon after its first appearance, is furnished by the large vessel of the hoop with a little artery and vein, probably formed in the same manner as the vessels of the tail and gills of the salamander already alluded to. The number and size pf the leaflets gradually augment, and, at the same time, each of the vascular arches is farther subdivided, till at last, when the structure of the gill is perfected, instead of the single arterial vessel, which at first winds round each hoop, there are formed a branchial artery and vein, the capillary vessels of wFich join by a multitude of minute ramifications on the surface of each of the leaflets. It has thus been shown by Rathke, that the branchial vein is originally a continuation of the trunk of the branchial artery. While those changes are taking place, the anterior vascular arch on each side, not contributing, like the posterior, to form the vessels of the gills, gives off an arterial branch (Fig. 1, at its anterior and convex part, which rises to the head, and corresponds with the carotid artery of higher animals. This vessel now no longer communicates with the • ascending aorta, but is supplied with blood by the posterior