282 ESSENTIALS OF ZOOLOGY 



intestine and stomach are thus promoted. The peritoneal 

 cavity is bounded anteriorly by the diaphragm, which com- 

 pletely separates the pleural cavities from the peritoneal 

 cavity. Each pleural sac consists of a parietal layer lining 

 the thorax and its moiety of the diaphragm. In the medial 

 plane it is reflected across the cavity of the thorax in 

 company with its neighbour to the bronchial tubes, where the 

 layers separate to pass in each case along the bronchus and 

 expand to cover the lung. The double membrane formed by 

 the two adposing medial parts of the pleura is called the 

 mediastinum, and in it lie the heart, the important vessels 

 near the heart, the oesophagus, nerves, and other structures 

 of the region (fig. 135). 



The pericardium is the similar membrane which surrounds 

 the heart. The inner or visceral layer is firmly attached to 

 the wall of the heart, and the outer or parietal layer leaves the 

 inner layer at the broad anterior end of the heart and comes 

 into close relationship with the mediastinum. The move- 

 ments of the lungs and heart are thus performed also by the 

 aid of smooth, moist, adposed surfaces. 



The rest of the mesoderm is concerned in forming the 

 connective tissues, the blood and blood-circulatory system, 

 and the lymphatic system. 



Vascular System. The blood consists of a fluid, the 

 plasma, in which are present red corpuscles or erythrocytes 

 in mammals these are not nucleated, the nucleus is absorbed 

 or expelled, or, according to some, the red corpuscles are the 

 liberated nuclei leucocytes, and thrombocytes. The ery- 

 throcytes, as has already been noted, are concerned in 

 respiration. The leucocytes, according to their situation and 

 difference in behaviour, are called by different names ; but they 

 are cells like all blood cells which are free, forming a sort of 

 nomadic tribe outside the direct government of the nerves, 

 though if they be thus outside authority, they are nevertheless 

 the most faithful servants the body possesses. They share in 

 absorbing food which might otherwise fail to reach the blood, 

 and in removing waste. They willingly sacrifice themselves, 

 if necessary, in waging war against all intruders, whether they 

 be living or dead, organic or inorganic. For all these purposes 



