64 ANATOMY OF THE RAT 



The left lung has but one lobe, the right has four. The 

 lobe of the left lung is larger than any of the lobes of the 

 right. Of the lobes of the right lung, three are in a row 

 on the right side of the thorax. Each of these has a convex 

 lateral and a concave median surface. The fourth, and 

 hindmost, lobe of the right lung is approximately pyram- 

 idal in shape. The concave base of the pyramid lies 

 against the diaphragm, the apex is directed toward the 

 heart. It lies for the most part at the left of the median 

 plane, and may therefore be mistaken for a lobe of the left 

 lung if its connection with the right bronchus is not dis- 

 closed. It is inclosed in the membranous sac previously 

 mentioned. 



Exercise XVII. Outline the ventral view of that part of the 

 dissected animal anterior to the diaphragm, and into this outline 

 draw the pleural eavityj lungs, esophagus, trachea, thyroid gland, 

 and larynx. 



Trace the left primary bronchus into the left lung. 

 Note how it divides into the secondary bronchi, these into 

 the tertiary bronchi, etc. Follow the subdivisions of the 

 right primary bronchus into the lobes of the right lung. 



The volume of the thoracic cavity is increased by the 

 periodic contraction of the muscles of the diaphragm and 

 thoracic wall. Atmospheric pressure then forces air into 

 the lungs through the nasal passages, trachea, and bronchi. 



Remove the larynx. In doing so injure the pharynx as 

 little as possible. The epiglottis is the flap attached to the 

 anteroventral border of the larynx. When food is being 

 swallowed the epiglottis folds back over the entrance into 

 the larynx, thus preventing the passage of food into 

 the trachea. The vocal cords are the pair of dorsoventral 

 folds in the larynx. The glottis is the slit between them 

 leading to the trachea. Dissect away the shreds of muscle 

 tissue adhering to the larynx, exposing the cartilages in 



