THE MUSCLES OF THE AIR PASSAGES. 45 



the broncho -trachea I 'is posticus, and the fasciculus / is the broncho- 



trachcalis anticus muscle ; b is a separate muscle, the broncho-trachealis 



brevis ; c, the bronchialis posticus ; e, the bronchialis anticus ; d, the 



stcrno-trachealis . (After Owen.) 

 FIG. 15. Lower larynx of a Parrot, a, tensor longus glottidis muscle. b, tensor 



brevis glottidis ; c, a narrow muscle passing from the tracheal to the 



bronchial half-rings. (After Owen.) 

 FIG. 16. Organ of hearing, Owl. /, the tensor tympani muscle ; c, the, Eustachian 



tube ; h', the superior semicircular canal ; c, tympanum, and b is its 



periphery. (After Owen.) 



28. The constrictor glottidis. --The inner margins of 

 the rima glottidis in the Raven are formed by the 

 mesial borders of the arytenoid bones. These latter 

 articulate posteriorly with the middle segment of the 

 cricoid, their anterior ends being free. Now the con- 

 strictor glottidis muscle arises, on either side, from the 

 superior and longitudinal line of the thyroid plate. 

 From this origin it sweeps outwards, then upwards, and 

 finally inwards in a gentle curve, to be inserted all along 

 the inner margin of the corresponding arytenoid bone, 

 and to the apex of the mid-cricoidal segment. 



When this pair of muscles firmly contract together, 

 they efficiently close the rima, and draw the apices of 

 the arytenoids to the middle line, and close against the 

 upper surface of the thyroid plate. This action of the 

 constrictor glottidis is opposed by the next muscle, the 

 opener of the rima. 



29. The tliyreo-arytenoideus (Figs. 10 and 18). --This 

 pair of muscles arise, one on either side, from the entire 

 outer margin of the thyroid plate, and the corresponding 

 margin of the outer cricoid bone (Fig. 10). 



The fibres pass inwards, directly across the opening of 

 the windpipe, to be inserted all along the outer margin 

 of the arytenoid bone of the same side, and the cor- 

 responding border of the central cricoid piece. It is 

 evident that the contraction of this pair will tend to 

 pull the arytenoid bones outwards, and thus open the 

 rima glottidis. 



