xiii.] THE FROG. 165 



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- 

 racter 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 abdomen be laid open, it will be found to inclose 

 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 

 those of the pleune and of the peritoneum in the higher 

 animals, it is termed the pleuroperitoneal cavity ; and the soft 

 smooth membrane which lines it and covers the contained 

 viscera is the pleuroperitoneal membrane. 



The vertebral column traverses the middle of the roof of 

 this cavity, and the layer of pleuroperitoneal membrane 

 which lines each lateral wall of the cavity, passes downwards 

 on each side of the vertebral column and joins its fellow in 

 the middle line to form a thin sheet, the mesentery, which 

 suspends the intestine. In the triangular interval left between 

 these two layers before they unite, a wide canal the sub- 

 vertebral lymph sinus the dorsal aorta, and the chain of 

 sympathetic ganglia, are situated. 



The dorsal moiety of the anterior end of the pleuroperi- 

 toneal cavity is occupied by the gullet, which places the 

 mouth in communication with the stomach. Beneath the 

 gullet the peritoneal cavity is separated only by a thin parti- 

 tion from a chamber, the pericardium, which contains the 

 heart. The posterior face of the partition is constituted by 



