104 Dr Allen Thomson on the Vascular System 



\y. On cutting out this opercular fold, the remains of the ex- 

 ternal gills may be seen for some time after they have been 

 enclosed, but they seem ultimately to be removed by absorption. 

 The internal gills which are in the mean time developed, are 

 by no means, as some have supposed, the enclosed external ap- 

 pendages, but are formed separately on the branchial hoops 

 encompassing the pharynx, like the gills of the adult fish. The 

 outer margin of these branchial hoops is gradually covered with 

 small processes of a soft substance that form the leaflets or 

 comb-like structure of the gills, upon which the minute capillary 

 vessels are afterwards ramified (Fig. 14, A), and these leaflets 

 are not unlike those of the external gills. The branchial hoops 

 are separated from one another by clefts, through which the 

 water introduced into the pharynx passes freely ; but after the 

 opercular fold has covered over the external and internal gills, 

 it unites with the integuments on the left side ; and only one aper- 

 ture, situated on the right side, is left, by which the whole of 

 the respired fluid makes its exit from the cavity of the gills *. 



As the vessels of the external gills are formed by the subdi- 

 vision of the simple branchial arches, and no branch is given oft' 

 from them before they arrive at the gills, the whole of the blood 

 which passes through the heart must necessarily be exposed in 

 these organs to the influence of the water, before it is sent to 

 nourish any other part of the body. 



The mode in which the subdivision of the branchial arteries 

 in the larva of the frog takes place has been observed by Rus- 

 coni, and is described by him in his anatomical description of 

 the larva of the aquatic salamander. 



Each branchial vascular arch, on entering its respective hoop, 

 gives off" a lateral branch, considerably larger than the continua- 

 tion of its own trunk : this lateral branch accompanies the pa- 

 rent vessel along the hoop, and reunites with it before leaving the 

 gill. As these two vessels proceed along the gill, side by side, 

 and at a short distance from one another, the lateral vessel gives 

 off* ten or more cross branches, which pass through the buds of 

 the leaflets, and fall again into the parent vessel. The lateral 

 branch, at its first separation very large, thus becomes gradually 



• In some species of frog, there is an opening on each side of the neck, as 

 in osseous fishes. See Cuvier's Recherches sur les Reptiles douteux. 



