I.] THE FROG. 7 



at least equal to, that of one of the upper eyelids. The 

 diameter of the tympanic membrane is less than that of the 

 eye, often much less. The horny elevation on the outer side 

 of the pes is small or absent, and that on the inner is 

 flattened and has a rounded margin. A patch of dark 

 colour extends from the eye backwards over the tympanic 

 membrane. The males have the cushion on the radial side 

 of the manus black, and they are devoid of vocal sacs. 



In Rana esculenta, on the other hand, the interspace be- 

 tween the eyes is usually concave and narrower than the 

 breadth of one of the eyelids. The diameter of the tym- 

 panic membrane is as great as that of the eye. The horny 

 elevation on the inner side of the pes is elongated, com- 

 pressed and brought to a blunt edge, so as almost to resem- 

 ble a spur, and a small outer elevation is constantly present. 

 There is no patch of colour at the sides of the head, such 

 as exists in Rana temporaria, and the cushion of the inner 

 digit in the male is not black. The males have a large 

 pouch on each side of the head, behind the angle of the 

 jaw, communicating with the cavity of the mouth, and, when 

 they croak, these pouches, becoming dilated, assume the 

 form of spherical sacs. 



Having thus become acquainted with the general cha- 

 racters and life-history of the Frog, and with those features 

 of its organization which are visible to the naked eye and 

 without dissection, its structure may next be studied in 

 detail. 



If the trunk be laid open, it will be found to enclose 

 a cavity in which some of the most important viscera — the 

 stomach and intestine, the liver, the pancreas, the spleen, 

 the lungs, the kidneys and urinary bladder, and the repro- 

 ductive organs — are contained. As this cavity answers to 



