132 ESSENTIALS OF ZOOLOGY 



of the larynx. They appear as folds of the epithelial lining. The 

 larynx which is similar to a small box continues into the tubular 

 trachea or windpipe posteriorly. The presence of a number of 

 C-shaped cartilages embedded in its wall prevents it from collapsing 

 as air is drawn through. The trachea extends parallel to, and is in 

 contact with, the ventral side of the esophagus. At the posterior end, 

 it bifurcates to form two primary h7'onchi, each of which leads to a 

 lung in which it is partially embedded. The left lung has but one lobe 

 while the right has four, three of which may be seen from the ventral 

 surface. 



Each lung is completely invested externally by a delicate, though 

 tough, transparent serous membrane called the pleura. This mem- 

 brane is continuous with the lining of the wall of the pleural cavity. 

 The portion adherent to the surface of the lung is the visceral pleura, 

 and the portion lining the inner surface of the body wall is the 

 parietal pleura. Within the lungs the primary bronchi branch and 

 continue to branch as secondary and tertiary bronchi, finally becom- 

 ing small hronchioles which lead into the alveoli, or tiny terminal 

 air sacs. 



Breathing is affected by increasing and decreasing the volume of 

 the thoracic cavity. To do this the ribs are moved forward and 

 spread by intercostal and other muscles, while the diaphragm which 

 usually arches anteriorly is contracted to a flat position. As the chest 

 cavity is thus enlarged, the internal pressure is reduced, and to bal- 

 ance the pressure, air naturally rushes into the lungs from the out- 

 side. When relaxed, the walls of the thorax and the diaphragm both 

 return to their original positions and expel the air. Drawing air into 

 the lungs is called inspiration, and discharging it is expiration. While 

 the air is in the alveoli of the lungs, oxygen is absorbed from it by 

 hemoglobin in the blood distributed in the capillaries of the pulmo- 

 nary veins embedded just beneath the epithelium here. At the same 

 time carbon dioxide is given up by the blood to the air. The pulmo- 

 nary veins return the oxygenated blood to the heart for redistribution 

 to all parts of the body. 



The Nervous System 



In the simple forms of life and in individual cells of the higher 

 forms, the cell membrane is the area where vital reaction takes 

 place. The two fundamental properties, irritability and conduc- 

 tivity, operate to receive the stimuli and conduct the impulses in the 



