220 THE LUNGS. 



left mass ; each of which presents deep fissures, partially 

 dividing its substance into what are called its lobes {Plate 

 VIII. /, g) . The right and largest lung presents four lobuli 

 {Plate VIII. h), the left three only. As already described, 

 the pleura first lines the thoracic cavity, and is then re- 

 flected over the lungs ; the lungs during life are in constant 

 motion, enlarging as the air is inspired, and diminishing as 

 it is expired ; whence we perceive the use of a serous mem- 

 brane within the chest : a second reflection from each lung 

 by a union of the membrane, forms an imperfect septum or 

 incomplete division ; the two lungs towards their centre 

 partially meeting, and being there only separated by a cel- 

 lular division. The colour of the lungs varies : in the 

 colt they present a pink hue ; in the adult horse they are 

 darker ; and in very old subjects they have a greyish cast 

 and granulated appearance. Internally they are always 

 dark in colour and spongy in texture ; being formed from 

 the ramifications of the bronchiee, which terminate in air- 

 cells ; and the bloodvessels which accompany them, all being 

 held together by an intervening cellular substance. 



The hronchicB are formed from the bifurcations of the 

 trachea (Plate VIII. A). They are composed of many 

 pieces of cartilage, connected together by fine ligament 

 {Plate VIII.) ; the cartilages becoming almost imperceptible 

 as the tubes grow more fine, or approach the air-cells. The 

 right and larger branch of the bronchiee given to the right 

 lung, quickly divides into three trunks. The left, which is 

 the longer, from the necessity it has to stretch itself under 

 the posterior aorta, divides into two principal trunks. The 

 bronchiae are lined throughout by the mucous membrane 

 continued from the larynx and trachea, furnishing a moisture 

 that loads the expired air or breath. The extreme ramifi- 

 cations of the bronchiae, as has been previously stated, end 

 in minute air-cells ; over whose surfaces are spread, in ex- 

 quisite minuteness, the capillary ramifications of the pulmo- 

 nary arteries, whose trunks accompany the bronchipe to the 

 lungs side by side. The pulmonic veins receive the blood 

 from the surface of the cells, whence it is by these last 

 vessels returned. The lungs are themselves nourished by 

 their appropriate arteries {FigVd . 5), whose blood is re- 

 turned by bronchial veins. Their nerves are furnished from 



