76 



in form, being compressed, conical, or triangular at its 

 lower surface, figs. 4 and 5, a and 6, of the second group, 

 having a central cross-bone, figs. 4 and 5, a a, extending 

 from behind to the front, dividing the orifice into two 

 equal parts ; to the outer side of which cross-bone the 

 inner membrane of each bronchial tube is attached. This 

 cross-bone, thus dividing the orifice, forms the point of 

 divarication from which the bronchiae arise separate, and 

 descend to the lungs. From the upper edge of this cross- 

 bone a semi -lunar shaped membrane, concave on its superior 

 edge, ascends for a short distance the inside of the tube. 



The bronchial tubes are formed on the outer sides by 

 membrane interposed between, and connecting a variable 

 number of cartilages which describe only parts of circles, 

 diminishing in size as they approach the lungs, fig. 2, c, 

 the circle being completed on the inner side by a delicate 

 membrane stretching from the opposite points of the 

 semi-circular cartilages, fig. 3, c, and forming a tube from 

 the orifice of the inferior larynx to the substance of the 

 lungs. This membrane is called by Cuvier the membrana 

 tympaniformis, and upon its dilatation and contraction, as 

 well as the power afforded of altering the form and length 

 of the bronchiae, some of the varieties of intonation depend. 

 The bronchiae are also slightly attached to each other, and 

 to the oesophagus. 



The muscles of the glottis, or superior larynx, are uni- 

 formly two pair in all the birds I have examined ; but the 

 muscles of the inferior, or true larynx, all largely supplied 

 with nerves, vary in number from one pair to five pair, 

 according to the genus or species, affording a corresponding 

 increase in the various qualities of the voice. 



Some few birds have no true muscles of voice at the 

 inferior part of their tracheae. The Vultures, some of 

 them at least, are described as being without any. 



