490 MR. YOU ATT* S VETERINARY LECTURES. 
function similar to that of the transverse muscle. Finally, al¬ 
though the triangular prolongation is found, but less developed, 
on the anterior or under side of the trachea at its termination, 
the additional plates of cartilage on the posterior or upper side are 
wanting, and for the same reason, because this portion of the tube 
is not subject to such occasional distention, and so much strength 
is not required. The number of rings in the ox is more than sixty. 
In the dog there are thirty-eight. In the swine forty. In the 
cat thirty-eight, with a vacuity between the extremities of the 
cartilage behind, and not forming a perfect ring. 
The Trachea in Birds .—In birds there is a singular variation 
in the form of the trachea. In some of them, and particularly in 
water-birds, it is immensely long; extending not only down the 
neck, but forming a circuitous course through the sternum, which 
is hollowed out to receive it, as you may perceive in this skeleton 
of the wild swan. The probable intention of this was to form a 
kind of reservoir of air to be gradually respired while the animal 
is diving. This, however, is found in greatest perfection in the 
male bird, and therefore is likewise connected with the voice; 
but, without this convolution, the length of the neck in birds 
would always render the trachea in them comparatively longer 
than in the mammalia. 
Fo long a tube requires to be more strongly constructed ; and 
therefore, with the exception of the two first, the rings of the tra¬ 
chea are mostly ossified ; the larynx also is entirely ossified. 
There is a peculiar muscular structure about the larynx and the os 
hyoides, which permits the bird to elongate the trachea to an 
extent no resemblance to which is found in the quadruped. 
The double Larynx .—The trachea of birds is, as you may 
observe in these various specimens and plates, curiously furnished 
with a double larynx, or rather a larynx divided into tw r o parts; 
one being placed at the superior, and the other at the inferior end 
of the tube. My friend Mr. Yarrell, in some excellent papers 
published in the Transactions of the Linnaean Society, has placed 
this in a plainer and more pleasing point of view than I have 
elsewdiere seen. The superior larynx consists of three small 
bones, answering to the posterior part of the cricoid cartilage ; a 
large bony plate, terminating in a point, and answering to the 
thyroid cartilage; and the true rima glottidis with the arytenoid 
bones, bordering it, and defending and influencing its motion, 
and furnished with muscles bearingconsiderable analogy to those 
of our other patients. There is no epiglottis or covering to the 
arytenoid bones and the rima glottidis, as in the quadruped, but 
the opening is surrounded by numerous papillae pointing back¬ 
wards, w 7 hich assist in directing and conveying the food towards 
