THE MUSCLES OF THE AIR PASSAGES. 47 



" From the simplicity of the structm'c just described, 

 from the situation of the superior hirynx with relation to 

 the rictus or gape of the bill, and from the absence of lips 

 by which this might be partially or entirely closed, it is 

 plain that it cannot be considered as influencing the voice, 

 otherwise than by dividing or articulating the notes after 

 they are formed by the lower larynx. The superior larynx 

 presents, indeed, but few varieties in the different species 

 of Birds ; and these relate chiefly to certain tubercles in 

 its anterior, which vary in number, and do not exist at 

 all in some species, as the Singing Birds ; being chiefly 

 present in those birds which have a rough, unmusical 

 voice. In the Pelican, the Gigantic Crane, and most of 

 the Rasoi'es, a process extends backward in the cavity 

 of the upper larynx from the middle of the posterior 

 surface of the thyroid cartilage, and seems destined to 

 give additional protection to the air-passages " (Owen). 



To examine the lower larynx, we must carefully free 

 the trachea from the strong; connective-tissue which sur- 

 rounds it between the limbs of the furcula ; then cut 

 through the clavicular arch close to the hypocleidium, 

 through the pectoral muscles, and down through the 

 sternal body close to the carina. By means of this 

 incision carefully open the chest. Next, after studying 

 the relations of the beautiful pair of sterno-trachealis 

 muscles, nip off the costal processes of the sternum, and 

 free them from attached structures. Cut througii the 

 bronchi close to the lung, on either side, then lift out the 

 lower larynx for examination. 



30. The traclieo -lateralis — or either one of a jDair of 

 muscles of that name, for all seven muscles of the lower 

 larynx of a Kaven are in pairs — is formed by the union 

 of the broncho-trachealis anticus and the broncho- 

 trachealis posticus, about one and a half centimetres 



