66 YERTEBRATA : MAMMALIA. 



the hum of flies, the stridulating noise of the grass- 

 hoppers, and the shrill of the crickets, are not vocal sounds, 

 but are noises which these animals make mainly with their 

 wings. 



The larynx is the organ of voice in Man and in all other mammals. 

 It is situated at the upper part of the trachea or wind-pipe, being sus- 

 pended from the bone of the tongue. It is formed of several carti- 

 laginous pieces, called respectively the thyroid cartilage, the cricoid 

 cartilage, and the arytaenoid cartilage. The most prominent part of 

 the thyroid in Man, is called Adam's apple. A mucous membrane 

 continuous with that of the mouth, nostrils, and pharynx, lines the 

 larynx, and, extending into the wind-pipe, becomes the mucous 

 membrane of the lungs themselves. In the interior of the larynx this 

 mucous membrane forms four folds two upper ones, called the false 

 vocal cords, and two lower ones, called the true weal cords, the space 

 between the latter being called the glottis. The vocal cords are under 

 the control of the will, and can be relaxed or tightened. The cavity 

 between the true and false .vocal cords is called the ventricle of the 

 larynx. As the passage of food or drink into the larynx is attended 

 with danger, producing suffocation unless speedily removed, the upper 

 aperture is protected by an organ of triangular shape called the epi- 

 glottis. 



In ordinary breathing, the air passes the vocal cords without pro- 

 ducing any sound ; but when they are drawn tighter, by the exercise 

 of the will of the animal, they narrow the aperture of the glottis, and 

 the air in passing them causes them to vibrate, and the vibrations pro- 

 duce the voice. The voice is modified according to the condition of 

 these cords, being higher when their tension is great, and lower when 

 their tension is less. 



Only one mammal Man has the power of making the 

 sounds of the voice into words ; he alone has the power of 

 true speech. 



As for Instinct, the Mammals are not more remarkable 

 than many other animals, as for example, Birds and Insects ; 

 but in Intelligence, as before stated, they are the highest 

 in rank of all the animals of the globe. 



The Class of Mammalia has been variously divided and 



