487 
ON THE TRACHEA. 
\ 
large the area of that passage. As the base is diminished, the 
area of the figure (unless the parts are expansible, which has not 
been proved to be the case with the anterior portion of the carti¬ 
laginous rings) must diminish too. Waving all this, I cannot 
conceive of the necessity of such an auxiliary for such a purpose. 
The force which can expel the air from the narrow and winding 
bronchial tubes, although acting there with so much disadvantage 
on account of the surrounding thickness of parenchymatous 
substance, wants no aid in propelling it through the unobstructed 
tube of the trachea. 
What theory have I to offer? None that I yet dare to urge 
with any great degree of confidence. I find, however, this mus¬ 
cular band at the base of the ellipse precisely at that part which 
the builder is most solicitous to strengthen. I find it, if not the 
buttress of the arch, yet the connecting tie by which it is bound 
together ; the chain which prevents it from spurring out at the 
bottom, where the principal stress lies. Then I naturally, I 
would almost say necessarily, connect this muscle with the idea 
of strength. It is the tie which is to prevent this arch from spur¬ 
ring out. In the natural state of the trachea, I can conceive this 
muscle to be, as it were, quiescent; but when any considerable 
pressure is made upon the crown of the arch at the upper part by 
tight reining, or at the lower by an ill-made collar, or any where 
else, by brutal or accidental violence, the moment the arch begins 
to be depressed at the crown, and consequently disposed to spur out 
at the base, this muscle contracts, and, by preventing the ex¬ 
pansion of the base, prevents also the further depression of the 
crown, and preserves the part from serious injury. 
Again; 1 can conceive that when, in violent exertion, so great 
an additional quantity of air is needed to answer to the increased 
demand for arterialized blood, and every portion of the respira¬ 
tory canal is on the stretch, this connecting self-acting band may 
preserve the trachea from injury, and possibly from laceration. 
The ligamentous connexion of the thin rings at the anterior part 
of the trachea might give way : and this at least I know, that, 
connected by cellular substance with, if not principally inserted 
into, the lining membrane of the trachea, the contraction of this 
muscle will tighten that membrane, will remove every inequality 
and corrugation, and so assist the passage of the air, both in in¬ 
spiration and expiration. This, however, will afford matter for 
the consideration of future veterinary physiologists. We have 
too many points yet to settle ; and we have been too shamefully 
indisposed to enter into the serious consideration o( them. 
The I'rachea within the Thorax. —Well, gentlemen, we trace 
the trachea down the neck until we arrive at the thorax; there it 
