96 PHYSIOLOGY AND TEMPERANCE. 



becomes what we may call an acquired singer; and lastly, 

 what is also of great importance, it moderates and improves 

 the quality of many voices which would otherwise be harsh 

 and unpleasant. 



Up to a certain period the pitch of the voice is much the 

 same in both sexes. About the age of fourteen the larynx 

 and vocal cords begin to grow rapidly, and the voice in 

 the boy " cracks." Frequently the cords grow so rapidly, 

 and become slack so suddenly, that a boy often wakens 

 in the morning to find his voice has changed. 



Voice is the sound produced by the vocal cords. Animals 

 can produce sound, but they cannot speak. Speech is the 

 voice modified by the mouth, tongue, teeth, lips and nose, 

 and is a faculty of the brain which belongs only to man. 

 Parrots and certain other birds have been taught to imitate 

 sounds, but they do not possess the true faculty of speech. In 

 whispering, the sounds are made by the vibration of the lips. 



G. The Pleura. The lungs are enveloped in a closely 

 attached serous membrane, called the pleura. Each lung has 

 a separate covering. This membrane is reflected from the 

 lung to the inner walls of the chest, and forms a complete 

 lining to the thoracic cavity. It is also a closed sac like 

 the other serous membranes of the body, and contains more 

 or less watery fluid. The regular expansion and contraction 

 of the chest causes a certain amount of motion between the 

 walls and the lungs. The smooth pleura, with the contained 

 fluid, assists the motion and prevents friction. Inflammation 

 of the pleura is called pleurisy. 



7. The Act of Breathing. The space within the chest 

 is increased on all sides by muscular action. One set of 

 the oblique muscles of the sides draws the ribs upward, 

 pressing the breast bone forward, while at the same time 

 the diaphragm descends against the contents of the abdo- 

 men. This enlargement of the chest cavity causes * rwh t> 



