Trnnsactiona of the Linnean Society. 259 



in succession. The superior larynx communicates with the mouth at the 

 root of the tongue, by a long and narrow orifice which is regulated as to 

 its extent of opening by two pairs of muscles, one of which is adapted 

 to close, and the other to dilate the glottis. By governing the size of the 

 aperture, these constitute one of the accessory means by which the sound 

 of the voice is regulated. The tube of the trachea varies in length, in 

 diameter, and in regularity, and the voice is influenced by each of these 

 variations ; thus shrill notes are produced by short trachea, low notes by 

 larger tubes, &c. Its substance, also has some effect on the voice; broad 

 cartilages usually coexisting with monotonous voices, while narrow rings 

 with enlarged membranous spaces allow freedom of motion, and conse- 

 quent variety of tone. 



The glottis and the trachea, however, only modify the voice, which is 

 truely produced by the inferior larynx. This part varies in form, in 

 structure, and in the number of its muscles. Its lower orifice is crossed 

 by a bone, which forms the point of divarication whence the bronchi 

 pass off to the lungs. The bronchi are composed of incomplete rings, 

 the circle being completed by a delicate membrane, the membrana lym- 

 paniformis. On the contraction and dilatation of this, and on the power 

 of altering the form and length of the bronchi, some of the varieties of 

 intonation depend. 



It is principally to the elucidation of the muscles of the inferior 

 larynx that Mr. Yarrell's observations are directed. These he considers 

 as the true muscles of voice. In some few birds, including the 

 Condor, the King of the Vultures, and the Spoonbill, they are entirely 

 wanting ; but they exist generally throughout the class, varying in number 

 from one pair to five pairs. A single pair is the number most usually met 

 with, being found, with very few exceptions, in all the Rasores, Gralla- 

 tores, JVatatores, and in some of the Insessores, as well as in the majority 

 of the Raptores. They arise from the whole outer surface of the cricoid 

 cartilage, and descending along the trachea, surround it at its upper part, 

 and afterwards divide and pass dovrawards in two equal portions attached 

 to the tube, which they do not quit till they have arrived at or near the 

 bone of divarication, when each passes off to be inserted upon the edge of 

 the sternum on its own side. These sterno-tracheal muscles influence 

 the length of the trachea as well as that of the bronchi. 



r2 



