RESPIRATORY ORGANS. 251 
B. Muscles on the Inner Surface of the Cartilages of the 
Larynx. 
M. thyreoarytenoideus (Fig. 105, @).—-The thyroarytenoid 
is a triangular, flat muscle of considerable size. It lies within 
the wing of the thyroid cartilage (2), and its fibres are nearly 
dorsoventral in direction. 
Origin.—The median longitudinal crest on the dorsal sur- 
face of the thyroid cartilage (2). 
Insertion.—The fibres converge to the insertion into the 
cranial lip of the laterocaudal angle of the arytenoid carti- 
lage (4). 
Action.—Turns the arytenoid on its oblique articulation so 
as to close the glottis. 
M. cricoarytenoideus lateralis (Fig. 105, c).—Triangular, 
a little smaller than the preceding, caudad of which it is 
situated, so that it also is covered by the wing of the thyroid. 
Origin.—The lateral part of the cranial border of the 
cricoid cartilage (3). The fibres converge, passing dorsad. 
Insertion.—The caudal lip of the laterocaudal angle of the 
arytenoid cartilage (4). 
Action.—Similar to the preceding, so that it closes the 
glottis. 
3. The Trachea (Fig. 105, 5; Fig. 106, a).—The trachea 
is that part of the air-passage which extends from the larynx 
to the bronchi (Fig. 106). It is a straight tube composed of a 
lining mucosa with ciliated epithelium, and a connective-tissue 
covering which encloses supporting cartilages. Each tracheal 
cartilage is incomplete dorsally where it lies- against the 
cesophagus, the gap between the two free ends of each ring 
being filled with muscular and connective tissue. As a result 
of this the diameter of the trachea is not fixed, but can be 
increased and diminished. The first ring is broader than the 
others. Where the cesophagus leaves the median line, the 
dorsal surface of the trachea lies against the longus colli mus- 
cles (Fig. 72, g’’). Its ventral surface is against the sterno- 
hyoid (Fig. 65, ¢) and sternothyroid (Fig. 65, g’) muscles. 
Its lateral surfaces are partly covered by the thyroid gland 
(Fig. 96, 6), and are in close relation with the carotid artery 
