152 



ELEMENTS OF PHYSIOLOGY 



a, network of capillaries; b t 

 small arteries and veins. 



of air cells. These capillaries are the termination of the 



branches of the pulmonary artery, and the beginning of the 



pulmonary veins. It is here that the 



blood changes from a purplish red to 



a rosy red. A fine connective tissue 



holds together all these air cells and 



tubes. 



254. The Lungs Entire. The en- 

 tire cavity of the chest except the 



space occupied by the heart and a few 



of its blood vessels and the esophagus, FlG I33 ._ The Blood Ves . 



is filled by the lungs and their cover- seis around TWO Air Ceils. 



ings. The lungs are light pink in early 



life but become grayish and darker 



as age advances. This change is more marked in persons 



who dwell in large cities or where the atmosphere is smoky 



and dusty. The lungs, or a part 

 of one, will float if thrown upon 

 water. The right lung has three 

 lobes, or divisions, and the left, 

 two lobes. The lower end, or 

 apex, of the heart hangs some- 

 what to the left, diminishing the 

 space for the left lung. The 

 general substance of the lungs 

 consists of bronchial tubes, blood 

 vessels, lymphatics, and air cells, 

 as above described, the air cells 

 being chiefly near the surface. 



FIG. i 34 . ciliated Ceils from the 255. The Pleuras. The sur- 



Trachea of a Rabbit, highly 



magnified. face of the lungs is not in con- 



m, m, m t mucous ceils in various tact with the chest wall. Imagine 



stages of secreting mucus. , c .. . 



a closed bag or sac made of thin 



membrane lining the whole of the chest. Now imagine 

 another closed sac a little smaller, that is inside of, and 



