AIR-TUBES AND LARYNX 371 



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 serves as the organ of voice, and appears first in, and 

 is restricted to, Birds. In the most usual form (broncJio-trackeal 

 syrinx}, there is a movable connection between the most anterior 

 bronchial rings, with which a complicated system of muscles is con- 

 nected ; these, by their contraction, cause a stretching or relaxing of 

 certain vibratory membranes. A bar of cartilage or bone, the 

 pessulus, extends from the junction of the bronchi into the more or 

 less swollen "tympanum" at the base of the trachea 1 : this supports 

 a slight fold of the mucous membrane called the memlrana semi- 

 lunaris, while the membranous inner wall of each bronchus is known 

 as the membrana tympaniformis internet: the external wall may 

 also give rise to a membrana tympaniformis extcrna. The tympanum, 

 which is strengthened by fused tracheal rings, 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 (Fig. 277). 



All the muscles of the syrinx are derived from the sterno- 

 hyoid group, i.e. from the cervical continuation of the rectus- 

 system : this is indicated by their innervation from hypoglossal 

 and cervical elements (Fig. 278). They are thus derivatives of the 

 trunk-muscles, and are therefore fundamentally different from 

 the laryngeal muscles, which have arisen by a modification of the 

 pharyngeal muscles, i.e. are of visceral origin.' 2 



The relative length of the trachea varies greatly in different 

 Birds, and its complete cartilaginous rings usually become calcified 

 or ossified. 3 



Mammals. The larynx of Mammals is distinguished from 



that of all other Vertebrates by the marked differentiation of the 



muscles the constrictors always exceeding the dilators in number, 



and by the constant presence of an epiglottis and a thyroid 



cartilage. 



The thyroid cartilage is derived from part of the fourth and 

 fifth visceral arches (cf. Fig. 279), and is closely connected with 

 the hyoid-apparatus : in Monotremes, in which alone it is paired, 

 this connection remains a close one. Apart from this fact, the 



1 In the Chelonian Cinixys homeana there is a somewhat similar " tym- 

 panum," the vocal cords are wanting in the larynx, and the laryngeal muscles 

 are reduced. 



2 Many differences are seen amongst the various avian groups as regards the 

 syringeal muscles. In some, both tracheo- bronchial and sterno- tracheal muscles 

 are wanting, while in others there may be as many as seven pairs. The syrinx 

 is simpler and more primitive in the female than in the male. 



3 In some cases (e.g. Swan, Crane), the windpipe 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 repre- 

 sentatives of the Sturnidse it extends between the skin and the muscles of the 

 thorax, and there gives rise to numerous spiral coils. 



B B* 



