492 Animal Biology 



quite meeting in the middle. Embedded in the edges of these folds are 

 fibrous and elastic ligaments which constitute the true vocal folds (vocal 

 cords), because they function in voice production as air passes between 

 them. Above the vocal folds are two smaller folds which do not aid in 

 voice production but protect the larynx during swallowing, help keep 

 the true vocal folds moist, and assist in holding the breath. They are 

 called the false vocal cords. The opening between the true vocal folds 

 is the glottis. The size of the glottis and the tension of the vocal folds 

 regulate the tone produced. The glottis is protected above by a leaf- 

 shaped fibrocartilage called the epiglottis. 



The trachea is a membranous tube about four inches long located in 

 front of the esophagus. The walls are strengthened by sixteen to twenty 

 cartilaginous, C-shaped structures. It extends from the lower end of the 

 larynx to the two branches, each of which is known as a bronchus (plural, 

 bronchi). Each bronchus divides and subdivides, the smallest branches 

 being the bronchioles. Each bronchiole terminates in a series of saclike 

 air cells (alveoli). The thin-walled alveoli are surrounded by thin-walled 

 capillaries through which the exchange of gases occurs. 



The two lu?igs are cone shaped and lie in the thorax (Figs. 235 and 

 243), being separated by the thick mediastinum which contains the heart, 

 larger blood vessels, trachea, etc. The left lung is smaller and longer 

 than the right because the heart occupies part of this space. Each lung 

 is enclosed in a serous sac called the pleura, which consists of an outer 

 layer, or parietal pleura, which adheres closely to the diaphragm and the 

 walls of the thorax, and a visceral pleura which covers the lungs. The 

 two pleurae are separated by a thin layer of serum to reduce friction. 

 Inflammation of the pleura is called pleurisy. 



VII. EXCRETION OF WASTES 



The excretion of human wastes may be considered as the elimination 

 from the body of the undesirable products of metabolism and other activi- 

 ties and includes liquids, gases, and solids (soluble and insoluble). The 

 elimination of indigestible materials which have served no purpose might 

 be considered as egestion. 



Excretory organs and the waste materials eliminated may be sum- 

 marized on page 493. 



The pair of bean-shaped kidneys, located at the back of the abdominal 

 cavity, one on each side of the vertebral column, select wastes from the 

 blood brought to them and pass them through the ureters to the urinary 



