286 COMPARATIVE ANATOMY 



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

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

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

 a slight fold of the mucous membrane called the membrana 

 semilunaris, 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 cxtcrna. The 



Tr 



FIG. 232. LARYNX OF MALE DUCK. (A, external, and B, internal view.) 



Tr, trachea; Br, bronchus; T, the "tympanum"; S, pessulus, from which 

 a lateral outgrowth (8 between & and 6) extends into the tympanum, 

 thus dividing its aperture into the trachea into two portions (6, 6) ; the 

 aperture is further diminished by the circular fold of mucous membrane, SF ; 

 t, thin region in S. 



tympanum 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. 232). 



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

 ginous rings usually 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 SturnicUe 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 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 branchial arches (comp. Fig. 233), and in Monotremes, in which 

 it is paired, it is still closely connected with the hyoid apparatus 



