STRUCTURE OF THE HORSE. 107 



are surrounded hy the pleurae, the smooth surface of the latter 

 diminishing friction during- the movements of respiration. They 

 consist of two conical-shaped, spongy organs, the right and left, 

 situated in the thoracic cavity. They are separated by the 

 mediastinum, heart, pericardium, and large blood vessels. The 

 right lung is larger than the left, and is more frequently dis- 

 eased. Healthy lungs float in water, and are of a rosy, flesh 

 color, marked by an irregular marbling or mottling. In conse- 

 quence of their extremely cellular or porous structure, they are 

 capable of great expansion and contraction during the process 

 of breathing. They consist of lobes, lobules, bronchi, terminal 

 bronchioles, alveolar passages and infundibula, air sacs, blood 

 vessels, and nerves. The lol)es are the primary divisions, the 

 right having three, the left two. The lobes are divided into 

 lobules of various sizes, separated by fine connective tissue. The 

 bronchi, on entering the lung, divide and redivide, each of a 

 smaller division entering a lobule. They resemble the trachea, 

 forming in the larger tubes incomplete cartilaginous rings by 

 sub-division. Each terminal bronchiole ends in one or more en- 

 larged passages, called the alveolar passages, from which are 

 given off blind dilatations, the end sacs. The air sacs, or cells, 

 are about 1-100 inch in diameter, lined by nucleated cells. The 

 pulmonary arteries accompany the bronchial tubes. Their ter- 

 minal branches lie between the air sacs, and send a net-work of 

 capillaries over them. The bronchial arteries arise from the 

 aorta, and are distributed to the bronchi, lymphatic glands, con- 

 nective tissue, and mucous membrane. The lymphatics arise 

 from the spaces occupied by the connective tissue cells in the 

 elastic tissue around the air sacs, and empty themselves into the 

 perivascular lymphatics, and eventually enter the bronchial 

 lymph glands. 



The nostrils furnish the sole means of admitting air to the 

 lungs. They modify the condition of the air. If it is too cold 

 they warm it; if too dry, they moisten it. They are lined by a 



