102 Dr Allen Thomson on the Vascular Si/stem 



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 (rs); 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- 



