106 Dr Allen Thomson on the Vascular St/stem 



obliterated when the gills have disappeared, so that the main 

 trunks only of the branchial arteries remain. These (the main 

 trunks) continue to carry blood to the arteries of the head, neck, 

 and anterior extremities (Fig. 15, t u). According to Rusconi, 

 the returning branches of the first, third, and fourth arches are 

 obliterated, those of the second only remaining to form the roots 

 of the aorta (r r'). The anterior extremities (Fig. 14,^) in the 

 mean time increase in size, and break through the skin which 

 covers them. The finny tail shrinks, and is gradually absorbed, 

 and the little frog now leaves the water to return only in search 

 of food or protection. 



Funk * and Siebold f have shewn, that as the batrachian rep- 

 tiles approach to maturity, another organ connected with the 

 respiratory function is developed, which seems calculated for ex- 

 posing a part of the blood at least to the influence of the oxy- 

 genizing medium. A vesicle formed by the extension of the 

 cloacal part of the intestine J becomes expanded so as to occupy 

 a considerable space at the posterior part of the abdomen ; the 

 umbilical arteries (Fig. 12, z) form a minute vascular network 

 by their subdivision on the surface of this vesicle, and the umbi- 

 lical vein (jk') returning from it conveys the blood which has 

 passed over its surface to the vena portae and liver, as in the 

 higher animals. This vesicle can only be considered as a rudi- 

 mentary and imperfect respiratory organ in the batrachian rep- 

 tiles ; but we shall afterwards find that it corresponds with a 

 part which becomes highly developed in the foetus of lizards, 

 birds, and mammalia, and forms in them a most powerful means 

 of effecting the respiratory changes in the blood. 



In speaking of the respiration of the foetus of batrachian rep- 

 tiles, it may be interesting to advert shortly to the structure 

 of the adult respiratory 'organs in some other animals, which ap- 

 pear to be nearly allied to this class, and are now very gene- 

 rally included by naturalists under the same general division. 

 The Proteus anguinus, the Siren lacertina, and the Amphi- 

 uma didactylusj and A. tridactylus, the species of those animals 



• De Salamandri Terrestri Formatione Vita et Evolutione. Berolini, 1827. 

 + Quaedam de Salamandris et Tritonibus. Berol. 1829. 

 t Seen commencing in the tadpole of the frog in Fig. 14^ v, and in the 

 adult salamander, Fig. 12. 



