THE BACK. 191 



lie in contact with one another, the pleural sac being prac- 

 tically obliterated, only producing sufficient serous fluid to 

 obviate any friction which might be caused b}^ the move- 

 ments of the parts. Thus any enlargement of the thorax 

 causes distension of the lungs, for air rushes in through the 

 trachea to fill up the extra space. In disease frequently 

 large quantities of fluid collect here, and membranes are 

 formed which, by causing union of the parietal to the 

 visceral portion of the pleura, lead to obliteration of the 

 sac, placing the animal in a condition analagous to that 

 normally obtaining in birds, which class of animals is 

 characterised by absence of pleura. Where the parietal 

 comes into connection with the visceral pleura the bronchi 

 and pulmonary arteries enter the lung, and the pulmonary 

 vems emerge from it. This is termed the root of the lung". 

 (The root of the left side is attached to the base of the 

 heart.) 



The trachea breaks up into two bronchi. The right is the 

 largest, and passes to the right, being crossed by the oeso- 

 phagus. It is larger than the left, and breaks up into three 

 parts on joining the right lung ; for this lung is larger 

 than the left, which is deficient centrally to allow room 

 for the movement of the apex of the heart in a direction 

 upwards and to the left. And it has also three lobes, while the 

 left has hut two. Between its middle and posterior lobes is 

 situated the pleural fold for the posterior vena cava and 

 right phrenic nerve. 



The bronchi are composed of complete cartilaginous 

 rings ; they divide and subdivide in the substance of the 

 lung, and the cartilaginous rings become thinner and divided 

 into several small portions until at last they disappear ; so 

 in the smaller bronchial tubes the walls are membranous. 

 Between the cartilaginous rings and the mucous membrane 

 are situated unstriated muscular fibres, extending in a cir- 

 cular manner comi^letely round the tubes. The mucous 

 membrane itself at first, like that of the trachea with which 

 it is directly continuous, presents columnar ciliated ej^i- 

 thelium ; but in the smallest bronchi the cells assume the 

 tessellated character and lose their ciliae, so that here it 

 resembles serous membrane. These minute bronchial tubes 

 terminate in the air-cells or infundibula, elongated spaces 

 with saculated ivalls (air-sacs), on the external surface of 

 which the pulmonary capillaries are distributed in so close 



