RESPIRATORY SYSTEM 67 
the walls of the long tubular proboscis, this apparatus is entirely 
wanting. 
Lravhea.—The narial passages have the organ of smell situated 
in their upper part, and communicate posteriorly with the 
pharynx, and through the glottis with the “trachea” or windpipe. 
a tube by which the air is conveyed to and from the lungs. The 
permanent patency of the trachea during the varied movements of 
the neck is provided for by its walls being stiffened by a series of 
cartilaginous rings or hoops, which in most mammals are incomplete 
behind. Having entered the thorax, the trachea bifurcates into the 
two bronchi, one of which enters. and, dividing dichotomously. 
ramifies through each lung. In some of the Cetacea and 
Artiodactyla a third bronchus is given off from the lower 
part of the trachea, above its bifureation, and enters the right 
lung. 
Larynzy—The upper end of the trachea is modified into the 
organ of voice or “larynx,” the air passing through which to and 
from the lungs is made use of to set the edges of the “vocal cords,” 
or fibrous bands stretched one on each side of the tube, into vibra- 
tion. The larynx is composed of several cartilages, such as the 
“thyroid,” the “cricoid,” and the “arytenoid” which are moved 
upon one another by muscles, and suspended from the hyoidean arch. 
By alteration of the relative position of these cartilages the cords 
can be tightened or relaxed, approximated or divaricated, as 
required to modulate the tone and volume of the voice. A median 
tongue-shaped fibro-cartilage at the top of the larynx. the “epiglottis,” 
protects the “ glottis.” or aperture by which the larynx communi- 
cates with the pharynx, from the entry of particles of food during 
deglutition. The form of the larynx and development of the vocal 
cords present many variations in different members of the class. 
the greatest modification from the ordinary type being met with in 
the Cetacea, where the arytenoid cartilages and epiglottis are united 
in a tubular manner, so as to project into the nasal passage, and, 
being grasped by the muscular posterior margin of the palate, pro- 
vide a direct channel of communication from the lungs to the 
external surface. An approach to this condition is met with in the 
Hippopotamus and some other Ungulates: it is indeed so general 
as an abnormality, that Howes suggests that an internarial epi- 
glottis may have been a primitive feature common throughout the 
class. Nearly all mammals have a voice, although sometimes it is 
only exercised at seasons of sexual excitement. Some Marsupials 
and Edentates appear to be quite mute. In no mammal is there 
an inferior larynx, or “ syrinx,” as in birds. 
Diaphragm.—tThe thoracie cavity of mammals differs from that 
of the Sauropsida in being completely separated from the abdomen 
by a muscular partition, the “diaphragm,” attached to the vertebral 
