8 ELEMENTARY BIOLOGY. [CHAP. 



those of the pleura 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 alimentary canal. In the triangular interval 

 left between these two layers before they unite, a wide 

 canal (subvertebral lymph sinus), the dorsal aorta and the 

 chain of sympathetic ganglia, are situated. 



The antero-dorsal moiety of the pleuroperitoneal cavity 

 is occupied by the gullet, which places the mouth in com- 

 munication with the stomach. Beneath the gullet the 

 pleuroperitoneal 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 

 pleuroperitoneum, 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 pleuroperitoneum lines the pleuroperitoneal 

 cavity and is reflected on to the intestine. The exposed 

 surfaces of both the pericardial and pleuroperitoneal mem- 

 branes are kept permanently moist by a fluid (serous fluid) 

 which more or less completely fills the cavities which they 

 enclose; hence they are termed serous membranes. 



A layer of the muscular fibres which enter into the body- 

 wall 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- 

 tuting a so-called diaphragm; which, it will be observed, is 



