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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    from without, as the deglutition of air, according to the obser¬ vations of the same author, is not seen to commence before the branchial apertures are closed at the period of transformation. In the progress of development, the lungs become more per¬ fectly formed, they are divided into compartments by the in¬ tersection of septa arising from their sides, and the air passages to the pharynx become more distinct. When the period of transformation approaches, a period varying much according to climate, season, situation, &c. the gills begin to shrink, and their vessels to carry less blood. The vessels ramified on the extremities of the leaflets become obliterated, and the parenchyma of the gill is forthwith ab¬ sorbed or removed. In about a week after this, so much of the gills has been removed that these organs are now reduced to mere tubercles projecting from the side of the neck. At the same time, the clefts, or branchial apertures into the pharynx, are gradually closed by the adhesion of an opercular fold of the integuments projecting from their anterior side, and the cartilaginous hoops which supported the gills become gradually softer and are removed. The respiration of the salamander [is now truly aerial, and this animal rises frequently to the surface of the water to expire and inspire air. While the transformation in the respiratory organs takes place, the bloodvessels distributed on them also undergo several important changes. The three anterior vascular branchial arches, which previously gave branches to the gills, are now relatively smaller than before (Fig. 11, m). The vessels which proceeded to and from the external gills are now wholly obliterated; their communicating branches at the root of the gill-stalks become dilated, and now form part of the continued trunks which wind round the pharynx, and join above it to form the descending aor¬ ta (r 5) ; so that the branchial arches of the pharynx seem to have resumed the simple form in which they first appeared before the external gills were developed. The arteries going to different parts of the head and neck, and arising from the outer parts of these vascular arches, are now proportionally larger than before. The fourth or posterior pair of arches {p) from which the lungs derive their arteries, and which was previously much smaller than the three anterior, now becomes the most consider-