ON THE TRACHEA. 
485 
it is essential that it should always be pervious, or at least that 
any obstruction to the process of respiration should be but mo¬ 
mentary. Attached to a part endowed with such extensive and 
varied motion as the neck, it is also necessary that it should be 
flexible. Can we find a substance firm, elastic, and yet flexible, 
to accomplish every desirable purpose? Cartilage is the most 
elastic substance we know, and at the same time possesses a cer¬ 
tain degree of flexibility. The trachea is composed of cartilage, 
but not of one entire piece, for that would necessarily be either 
too thick and firm to be flexible, or, if it were sufficiently flexible 
to accommodate itself to the motion of the neck, it would be too 
weak to resist even common pressure or injury, and the passage 
through it would often be inconveniently or dangerously ob¬ 
structed. Beside, it is necessary that this tube should occa¬ 
sionally admit of elongation to a considerable degree. When the 
neck is extended in the act of grazing or otherwise, the trachea 
must be somewhat lengthened. 
The structure of the cartilage of the windpipe is admirably 
adapted to effect every purpose. It is divided into rings, 50 or 
52 in number, each possessing sufficient thickness and strength to 
resist ordinary pressure; and each constituting a joint with the one 
above and below, and thus admitting of all the flexibility that can 
be required. These rings are connected together by an interposed 
fibro-ligamentous substance, extensible, elastic, and yet so strong 
that it is scarcely possible to rupture it; and the fibres of this liga¬ 
ment not running vertically from one ring to another, and so ad¬ 
mitting of little more motion than the nutation of the head, but 
composed of two layers running obliquely and in contrary direc¬ 
tions, so as to adapt themselves to every possible variety of motion. 
Take any two rings, and see the extensibility of the ligament, 
the freedom of motion which it gives in every direction, and the 
rapidity and recoil with which, when the force that separates 
them is removed, they return to their natural state of approxi¬ 
mation. 
These rings are thickest in front, and project circularly, oppos¬ 
ing an arch-like form. There, too, the ligament is widest, to 
admit of the greatest motion in the direction in which it is most 
needed, when the head is elevated or depressed. Laterally these 
rings are thinner, because they are to a great degree protected by 
the surrounding parts ; and posteriorly, as you see, they overlap 
each other, and that extensively, but in an apparently irregular 
manner; and the overlapping portions or plates are connected 
together by a strong ligamentous substance. This, while it docs 
not impede the motion of the tube, gives firmness and stability 
to it. You will immediately perceive, that, if the fibro-ligamen- 
