160 ELEMENTS OF MAMMALIAN ANATOMY. 
vocal cords. These are of two kinds—true and false. 
They are best demonstrated by dividing the larynx, sagit- 
tally. The superior or false vocal cords are the two 
superior thyro-arytenoid ligaments passing from the aryte- 
noid to the thyroid cartilage, at the base of the epiglottis. 
The cat is said to use these cords in purring. The in- 
ferior or true vocal cords are the inferior thyro-arytenoid 
ligaments, consisting, as in man, of a fold of mucous mem- 
brane on either side, just caudad to the false vocal cords 
(Fig. 56). 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 thyroid cartilages, 
the tenseness of the vocal cords is regulated, and the 
various pitches of voice produced. 
The trachea, or windpipe, is the tube leading from the 
larynx to the lungs (Fig. 55). Its walls are prevented 
from collapsing by the presence of about forty-five carti- 
laginous rings which are incomplete dorsally. Before pass- 
ing into the lungs the trachea bifurcates into a right and a 
left bronchus. The trachea and bronchi are lined by 
ciliated columnar cpithelium. 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. Ciliated 
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 
