158 ELEMENTARY BIOLOGY. [xill. 



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 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 

 those of the pleurae 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 sus- 

 pends the intestine. In the triangular interval left between 

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

 bral lymph sinus the dorsal aorta, and the chain of sympa- 

 thetic 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 partition 

 from a chamber, the pericardium, which contains the heart. 

 The posterior face of the partition is constituted by the peri- 

 toneum, its anterior face by a membrane of similar character, 

 the pericardial membrane, which lines the pericardium and 

 is reflected on to the heart, in the same way as the peritoneum 

 lines the peritoneal cavity and is reflected on to the intestine. 



A layer of the muscular fibres which enter into the walls 

 of the abdomen, is continued inwards at the anterior boundary 

 of the pleuroperitoneal cavity and is attached to the sides of 

 the oesophagus and to those of the pericardium, thus consti- 



