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 

 oesophagus, 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 oesophagus 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, e) 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 



