124 



THE AXAT0MY OF THE HORSE. 



divided by delicate intersecting lines into small areas, each of which 

 corresponds to a lobule of the lung. The lines are formed by the inter- 

 lobular connective-tissue, which in the horse is very sparing in 

 amount. 



The colour of the lung varies with the age of the animal. 

 In the young subject it is pale pink, but in old animals it has 



a slight grayish or slaty 

 tinge. In the foetus it is 

 a bright pink. 



The lung is spongy to the 

 touch, and it is also markedly 

 elastic, this quality being best 

 illustrated by the rapidity 

 with which the inflated lung 

 collapses when the distending 

 force is removed. It crepi- 

 tates on pressure with the 

 fingers, and it floats on 

 water. The foetal lung is 

 non crepitant, and sinks in 

 water. 



Structure. — When the bron- 

 chus enters the lung, it divides 

 again and again until there 

 results a remarkable tree-like 

 arrangement of bronchial' 

 tubes. Of this tree, the bron- 

 chus entering the root of the 

 lung forms the main stem ; 

 and, as the division is traced 

 onwards, the bronchial tubes, 

 representing the branches, become smaller and smaller, until there 

 is reached a tube of comparatively small calibre which belongs 

 exclusively to one lobule, and is therefore termed a lobular or terminal 

 bronchus. The left bronchus has a length of three or four inches before 

 dividing, but the right immediately gives off from its outer side a con- 

 siderable branch (Plate 26). Within each lobule the terminal bronchus 

 ramifies, forming smaller tubes or bronchioles, the last and smallest of 

 which lead into cavities termed alveolar passages. These are bounded 

 by delicate sacculated walls, each sacculation being termed an air-cell 

 or alveolus. The air-cell is thus the ultimate part of the air passages 

 within the lung, and its wall consists of a delicate membrane support- 

 ing the capillary plexus of the pulmonary vessels, and lined towards the 



Fig. 8. 



termination (if thk alr passages in thk 

 Lung (modified from Turner). 



A. A. Terminal bronchi 

 C. Pulmonary artery ; I). 

 E. Pulmonary capillaries. 



B. An air-cell ; 

 Pulmonary vein ; 



