194 ELEMENTS OF MAMMALIAN ANATOMY 



enoid ligaments passing from the arytenoid to the thyreoid 

 cartilage, at the base of the epiglottis. The cat is said to 

 use these cords in purring. The inferior or true vocal cords 

 are the inferior thyrco-arytenoid ligaments, consisting, as in 

 man, of a fold of mucous membrane on either side, just caudad 

 to the false vocal cords (Fig. 62). The depression on either 

 side between the true and false vocal cords is the ventricle. 

 By means of the muscles attached to the arytenoid and thy- 

 reoid cartilages, the tenseness of the vocal cord is regulated, 

 and the various pitches of voice produced. 



The trachea, or windpipe, is the tube leading from the larynx 

 to the lungs (Fig. 61). Its walls are prevented from collapsing 

 by the presence of about forty-five cartilaginous rings which 

 are incomplete dorsally. Before passing into the lungs the 

 trachea bifurcates into a right and a left bronchus. The 

 trachea and bronchi are lined by ciliated columnar epithelium. 

 The cilia project freely into the lumen from the columnar walls, 

 and are always waving in such a manner as to carry the secreted 

 mucus lodged thereon toward the mouth, thereby preventing 

 the lungs from becoming clogged with foreign material. Cili- 

 ated cells with the cilia in action may be easily demonstrated 

 by scraping very lightly a little mucus from the posterior part of 

 the roof of the frog's mouth, and mounting the same in a drop of 

 saliva on a glass slip, which is then to be examined with a 

 microscope magnifying about 300 diameters. The diaphragm 

 of the microscope should be arranged so as to admit but little 

 light. 



The lungs together with the heart fill up the greater part of 

 the thoracic cavity. In a cat recently killed the lungs may be 

 expanded by tying a piece of glass tubing on the trachea and 

 blowing into it strongly for a few seconds. Each lung is com- 

 pletely invested by a sac of delicate transparent serous mem- 

 brane called pleura (Fig. 93). Each sac is reflected at the root 

 of the lung, where the blood-vessels and bronchus enter, so as 

 to form a parietal layer lining its half of the thoracic cavity. 



