Transactions 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 ¢rachea 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 tym- 
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 
larynz 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, inall the Rasores, Gralla- 
tores, Natatores, 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 downwards 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 
