RESPIRATION 



519 



encloses the heart (/. vent, r. vent\ the mediastinum here 

 fusing with the visceral layer of the pericardium (vis. per) 

 and thus obliterating the space ; below this it again narrows 

 to form the ventral mediastinal space (v. med\ in which the 

 thymus (p. 507) is situated. 



In the entire animal, the air-tight pleural cavities are 



cent 



FIG. 138. Diagrammatic transverse section of the rabbit's thorax in the region of 

 the ventricles, to show the relations of the pleurae and mediastinum (dotted line), 

 etc. The lungs are contracted. 



aort. dorsal aorta ; az. -v. azygos vein ; cent, centrum of thoracic vertebra ; /. Ing. 

 left lung ; I. pi. left pleural cavity ; /. vent, left ventricle ; my. spinal cord ; oes. 



v. med. ventral mediastinal space. (From Parker and Haswell's Zoology.) 



completely filled by the lungs, so that the parietal and vis- 

 ceral layers of the pleurae are practically in contact, there 

 being only a lubricating serous fluid (lymph) between them. 

 The pressure of the air in the bronchial cavities of the 

 lungs is therefore sufficient to keep them distended ; but as 



