ON THE TRACHEA OF THE OX. 
489 
tached to it, and presents an uncertain number (from ten to 
fifteen) of plicae or folds, running longitudinally from the larynx to 
the bronchial bifurcation. These evidently prove that the trachea 
* is capable of dilatation, probably when severe exertion is required 
from the animal. The transverse muscle is not concerned in 
this dilatation, for the action of a muscle is to contract; but it 
yields more readily than any other portion of the tube, or is, in 
fact, the only part that yields. When the muscle has yielded 
or been distended to a certain degree, it begins to contract, and 
limits the augmentation of the tube; or while the tube is dilating 
it offers a certain resistance, which, if it does not altogether pre¬ 
vent the augmentation of calibre, yet considerably restrains it. 
The ligamentous connexion of the plates of cartilage behind 
would not do this so effectually or so safely. 
The Bloodvessels and Nerves .—The arteries of the trachea 
are a branch of the thyroideal at the upper part, and below are 
various ramifications from the ascending common trunk of the 
carotid. The lowest part is supplied from the posterior aorta, 
and principally from the bronchial arteries. The veins of the 
trachea empty themselves by several small outlets into the jugu¬ 
lar. The nerves above the thorax are chiefly derived from the 
ascending branch of the recurrent nerve. Within the chest, they 
are obtained more from the great organic nerve than from the 
eighth (par vagum), and the union of many filaments from both of 
them forms the tracheal plexus, clinging around the trachea 
between the first and second ribs. 
The Trachea in other Animals .—The trachea of the ox is pro¬ 
portionally much smaller than in the horse ; so much air is not 
wanted, because he is not a beast of speed, and rarely goes be¬ 
yond the walk or trot. The cartilaginous rings are narrower, and 
considerably thicker, and of less firm consistence, than in the 
horse, and the interposed ligamentous substance wider. A tube 
of looser construction will be sufficient for the portion of air 
which he needs in respiration ; and, gathering usually the whole 
of his food from the ground, and gathering it more slowly, and 
being longer occupied about it, a greater degree of extension is 
required, and therefore more interposed ligament is given. There 
is not the careful and intricate overlapping of the cartilages 
behind; they are simply brought into approximation with each 
other, and their opposing edges project behind, so that the 
trachea is less firmly bound to the cervical vertebrae ; nor is there 
any transverse muscle, because the calibre of the tube is not so 
much varied as it is in the rapid progression of the horse; but 
the lining membrane is denser, and more extensible, and more 
elastic, and is capable of discharging, although imperfectly, a 
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