TRACHEA OF THE ROOK. 25 



of the basal half of the arytenoid bones. Their lower or pos- 

 terior fibres are transverse, and they may be considered as ana- 

 logous to the arytenoldei-ohllqid and traiwcer sales in man. 



4. Beneath the thyro-arytenoidei, Fig. 9, which open the 

 glottis, are a pair of muscles, the thyro-cricoidei^ Fig. 11, 

 which arising from the inner edge of the upper and back part 

 of the thyroid bone and its appendage, are inserted into the 

 outer side of the basal half of the arytenoid bones, and into the 

 back part of the cricoid. Their action is to assist in closing 

 the glottis, but more especially to elevate and draw forward 

 the cricoid and arytenoid bones. 



5. Besides these may be mentioned a pair of very small 

 oblique muscles, seen in Fig. 11, and more distinctly in Fig. 

 12, which pass from the edge of the appendages of the thyroid 

 bone to be inserted into the cricoid and arytenoid, and there- 

 fore may be named thyro-cricoidei postici. Their action is to 

 pull the cricoid and arytenoid bones backward and downward. 



Having thus examined the larynx, or upper extremity of the 

 trachea, we continue our inspection, observing that the body of 

 that tube, Fig. 1, o jt> 5', is of nearly uniform breadth, until at its 

 lower extremity, q, where it is much contracted, and that its 

 rings are generally narrower in the median line than at the sides. 



From each side of the larynx arises a thin muscle, s s, 

 which soon becomes narrower, and extends along the trachea 

 in its whole length, until it is blended with the muscles situ- 

 ated on the syrinx, q. This muscle is generally named the 

 sterno-trachealis^ because it usually leaves the trachea at its 

 lower part, and passes backward to be inserted into the ante- 

 rior portion of the costal process of the sternum. In some 

 birds, as the Ducks, there are two such muscles on each side : 

 in other words, the cleido-trachealis, already described, in- 

 stead of separating from the trachea at its commencement, 

 adheres to it until near the furcula. Here, however, it may 

 with more propriety be considered as terminating at the lower 

 extremity of the trachea ; and, as its action is to contract or 

 shorten the tube, it may be named contractor trachew. 



At the bifurcation, q, the tube of the trachea contracts, and 

 becomes circular, in place of being flattened. One of its last 



