THE RESPIRATORY SYSTEM 1 95 



The median space between the two sacs is called the media- 

 stinum. The anterior or ventral mediastinum contains blood- 

 vessels and the thymus gland. The dorsal or posterior medi- 

 astinum lodges the trachea, esophagus and aorta and the heart 

 occupies the middle mediastinum. 



Each lung is divided by deep clefts into several lobes. The 

 left lung is composed of two large lobes and a small one. The 



Fig. 95. — Diagram Showing Structure of the Lung on the Cat. (Modified 



after Miller.) 



A, Artery; B, bronchiolus; B.R, respiratory bronchiole; D.A, alveolar duct; a, 



atria; A.S, alveolar sac; A.P, pulmonary alveolus; V, vein. 



right lung consists of four unequal lobes. The cranial end of 

 the lung is the apex and the caudal end, resting against the dia- 

 phragm, is the base. The bronchi, as they are continued into 

 the lungs, subdivide into smaller tubes, whose later subdivisions 

 are the bronchioles. The latter, dividing like the branches of 

 a tree, finally terminate in blind pouches known as infundihula 

 or alveoli, the wall of which are thickly beset with microscopic 



