AIR-PASSAGES. 255 



No considerable advance in structure is, however, seen ; there is 

 even a reduction noticeable as regards the musculature, for as a 

 rule, only a single dilator and constrictor are present (Fig. 207, 

 D, S l ), instead of several, as in the Frog. 



One point, however, must be specially noticed, viz., the close 

 connection which obtains between the larynx and the hyoidean 

 apparatus more particularly the dorsal surface of the basi-hyal. 

 In Crocodiles and Chelonians, for instance, the larynx is 

 firmly embedded in a shallow depression of the latter, and it seems 

 probable that the thyroid cartilage of Mammals has been 

 derived from a part of the body of the hyoid. 



A well-developed trachea, always supported by enclosing 

 cartilages, is present in all Reptiles, but the cartilages are not in 

 all cases fused together to form complete rings. The walls of the 

 bronchi are also usually provided with cartilaginous supports. 



Birds. In Birds there are two larynges, an upper and a 

 lower. The former lies in the usual position behind the tongue on 

 the floor of the pharynx, and is plainly homologous with that of 

 other Vertebrates, though it is incapable of producing sound. 

 This is owing to the fact that both its skeleton and muscles are 

 obviously undergoing a retrogressive metamorphosis. 



The lower larynx, or syrinx, is of much greater interest ; it is 

 usually situated at the junction of the trachea and bronchi, or more 

 seldom at the lower end of the trachea alone or on the bronchi 

 alone. It functions as the organ of voice, and appears first in, 

 and is restricted to, Birds, no traces of a syrinx, which might be 

 expected, being found in Reptiles. In the most usual form, or 

 broncho-tracheal syrinx, there is a moveable connection 

 between the most anterior bronchial rings, with which a compli- 

 cated system of muscles is connected ; these, by their contraction, 

 cause a stretching or relaxing of certain vibratory membranes 

 (membrana tympaniformis interna and externa). The 

 lower specially modified end of the trachea also plays an impor- 

 tant part as a " tympanum," which attains a relatively enormous 

 development in some Water-Birds (e.g., the male Duck), where it 

 gives rise to a bony vesicle which serves as a resonance cavity. 



The length of the trachea in Birds varies greatly, and its complete cartila- 

 ginous rings show a great tendency to become ossified. In some cases (e.g. 

 the Swan and Crane) it extends into the hollow keel of the sternum, where it 

 becomes more or less coiled, and then again passes out close to its point of 

 entrance, and enters the body-cavity. In certain representatives of the family 

 of Sturnidse it extends between the skin and the muscles of the thorax, and 

 there gives rise to numerous spiral coils. 



Mammals. The larynx of Mammals is distinguished from 

 that of all other Vertebrates by the following characters : there is 

 always a very marked differentiation of the muscles, the 

 constrictors always exceeding the dilators in number; and an 

 epiglottis and a thyroid cartilage are constantly present. 



