THE CONSTRUCTION OF THE HORSE 141 



vening connective tissue. It is commonly the seat of 

 disease, particularly in *' roaring/' The trachea is 

 composed of a number of cartilaginous rings united 

 together to form a single tube, which, as it enters the 

 chest, bifurcates into the right and left bronchial tubes, 

 each of which passes to the lungs, again divides and sub- 

 divides until, finally, its ramifications, plus blood-vessels, 

 fibrous tissue, etc., form the lungs. 



The trachea in the horse is close to the surface of the 

 skin throughout a considerable portion of its course, 

 consequently access to its interior, in other words, 

 tracheotomy, is an operation comparatively easy to 

 perform. 



The lungs are large and occupy the bulk of the cavity 

 of the chest ; the heart and the great vessels are suspended 

 between the two. Each lung is covered by a delicate 

 glistening membrane known as the pleural membrane, 

 and this is also reflected over the inner side of the ribs to 

 form the costal pleura in contradistinction to that cover- 

 ing the lungs, the pulmonary pleura. This serous mem- 

 brane is also reflected over the heart, forming the peri- 

 cardium in which the heart is enclosed. 



The lungs have a very rich blood supply, and the three 

 principal vessels are the pulmonary artery, the pulmonary 

 vein, and the bronchial artery. Unlike other vessels in 

 the body the pulmonary artery carries impure blood, 

 and the pulm.onary vein arterial blood, but the nutrient 

 aitery of the lung is the bronchial. These features, 

 however, are of merely passing interest to the reader of 

 this book. 



During the act of inspiration pure air is admitted into 

 the lungs by the trachea, and it passes into the pulmonary 

 alveoli which are encircled by the capillaries, containing 

 impure blood, for oxidation during the act of inspiration. 

 The expiratory effort comprises the discharge of carbon 

 dioxide from the lungs after purification of the blood in 

 these structures. 



