CH. VIIl] FKOGS, TOADS, AND NEWTS 261 



within them is probably an important factor in keeping 

 the skin moist and in a fit state for the interchange of 

 gases in respiration. There are numerous minute openings 

 between these subcutaneous lymph sacs and the adjacent 

 lymphatics of the underlying organs. In connexion with 

 the vessels are two pairs of small contractile sacs, the 

 anterior and posterior lymph-hearts : the former lie behind 

 the transverse processes of the third vertebra and by their 

 rhythmic contractions, assisted by sundry valves, drive 

 lymph into the vertebral vein; the latter are placed on 

 either side of the hinder end of the urostyle the rod of 

 bone which continues the line of the vertebral column 

 and pour their contents into a vein that connects the 

 femoral and iliac veins near the top of the thigh. The 

 lymph itself is a clear transparent fluid containing colour- 

 less corpuscles, identical with those of the blood. It is 

 virtually blood minus red corpuscles. Thus all the solid 

 tissues of the body are constantly bathed in fluids bringing 

 to them from the respiratory and digestive organs oxygen 

 and nutrient material, and removing carbon dioxide and 

 other waste products which are conveyed away to the 

 excretory organs, and by them cast out of the body. 



Respiration 1 . The respiratory mechanism is one of 

 considerable complexity. The absence of diaphragm and 

 of ribs at once shows that inspiration and expiration are 

 carried on by means very different from those found in 

 ourselves. Moreover the skin of a frog is thin and 

 constantly moist, so that much of the interchange of gases 



1 Gaupp, Arch. f. Anat. und Phys. (Anat. Abth.), 1896 ; Baglioni, ibid. 

 (Physiol. Abth.), 1900, Supp. Ed. 



