EESPIKATION. 463 



anterior of which extends across in front of the heart. In 

 the dog, the left has three lobes and the right four, the divi- 

 sions extending, in this case, nearly to the roots of the lungs. 

 It is noticeable that each lobe receives a single large bronchial 

 tube, each group of lobules a special bronchial tube of smaller 

 caliber, each lobule a still more diminutive special branch, 

 while each ultimate lobulette has its own terminal bronchus. 



The interlobular connective tissue predominates most in 

 young animals; its amount varies likewise in the different 

 species, being especially abundant in the lungs of ruminants. 



The lungs, as a whole, have a somewhat conical form, the 

 rounded apex being presented toward the neck, while the 

 irregularly concave base is turned backward, and applied 

 against the diaphragm. The right lung is generally larger 

 than the left, on account of the position of the heart being 

 slightly to the left side of the median line. 



Each lung is covered by a special serous membrane 

 (pleura), which envelopes the lung, and is reflected on the 

 walls of the thorax. Each pleura is divided into four por- 

 tions the pulmonic or visceral, the mediastinal, the dia- 

 phragmatic, and the parietal. The pulmonic pleura covers 

 the whole external aspect of the lungs, to the parenchymatous 

 structure of which it is intimately attached by a layer of 

 elastic tissue. This connecting layer dips into the pulmonary 

 'tissue, becoming continuous with the interlobular layer. 

 Each pleura passes off the lungs at the bronchial tubes, and 

 becomes continuous with the mediastinal pleura. The 

 latter is an extensive membrane attached to the sides of the 

 vertebrae above, the sternum below, and the middle of the 

 diaphragm behind. The mediastinal pleurae on both sides thus 

 concur in forming a complete median partition for the thorax, 

 but in the horse their structure is so lax, postero-inferiorly, that 

 liquid readily passes from the pleura on one side to that on 



