CIRCULATION. 



645 



Fig. 317. 



Frog. 



fined surface for the distribution of the pulmo- 

 nary capillary vessels. 



In the aquatc Reptiles having gills, such as 

 the larvae of the Frogs and Salamanders in their 

 transitory conditions, and the Protean animals, 



Fig. 318. 



Proteus Mexicanus (Axolotl). 



which are very similar to them, but do not un- 

 dergo, so far as is known, any further metamor- 

 phoses, the branchial organs are formed by an 

 extension or minute subdivision of branches of 

 the aortic trunk, supported upon the arches of 

 the hyoid bones. In all of these Reptiles, the 

 ventricle consists of a single cavity (Jig. 318, 

 Ji), which propels its blood into the bulb or 

 commencement of the aortic trunk (A ). The 

 aortic trunk divides into two branches, each of 

 which subdivides again into three or four ves- 

 sels upon each side of the neck. These vessels 

 ( B ), passing round the gullet or upper part of 

 the alimentary canal in the form of lateral 

 arches, unite again together behind, to form the 

 descending aorta. The branchial apparatus of 

 the animals now under consideration is formed 

 entirely upon these lateral arches of the aortic 

 trunk. In the larva of the Salamanders, in 

 the Proteus, Axolotl, Menobranchus, and 

 Siren,* the small branches of each gill are 

 formed by the minute subdivision of a loop of 

 vessel prolonged from the outer part of three of 

 the arches on each side into leafed processes of 

 the cuticular system attached to the hyoid ar- 

 ches ( B, 6). 



The larva of the Frog has, in the earliest 

 stage of its existence, gills of the same kind as 

 those just described; but in its more advanced 

 condition these external gills disappear, and 

 the larva of the frog breathes by internal gills 

 more resembling those of fishes than the ex- 

 ternal branchiae of the Newt or Proteus. The 

 gills of the tadpole of the Frog are covered 

 by the skin, and consist of a great number of 

 small leaflets, receiving the minutely subdi- 

 vided loops of vessel given off for some way 

 along each of the four vascular arches as they 

 pass round the neck along the cartilaginous 

 hoops of the hyoid bone. The vascular arches 

 are double in that part of their course where 

 they are connected with the gill, the blood 

 being transmitted from one branch to the other 

 in passing through the leaflets of the gill. 



In the larvae of the Batrachia, from a very early 

 period of their existence, as well as in the Pro- 

 tean Reptiles, there are lungs which seem to 

 be used as adjuvant respiratory organs, for they 

 are generally filled by the animal with air from 

 time to time. These lungs, more or less per- 

 fectly developed in different kinds of Protean 

 Reptiles, and at different stages of the existence 

 of the Batrachian larvae, all receive a pulmo- 

 nary vessel from the vascular arch of the aorta 

 which is nearest the heart, whether this arch is 

 connected with a branchial apparatus or not. 



In all these animals the anatomical relations 

 and the mode of development of the blood- 

 vessels of the gills proves distinctly their re- 

 turning vessels to be, as much as those which 

 conduct the blood into them, branches of the 

 arterial system; but the lungs on the other 

 hand, however rudimentary, are almost always 

 furnished with proper pulmonary veins which 

 lead to the auricle of the heart. 



The following is the course which the blood 

 takes in this interesting class of animals. The 



* We omit the consideration of the Amphiiima, 

 Menopoma, and Cceiilia. 



