THE TRACHEA. 1111 



The first cartilage is broader than the rest, and sometimes divided at one end ; 

 it is connected by fibrous membrane with the lower border of the cricoid cartilage, 

 with which or with the succeeding cartilage it is sometimes blended. 



The last cartilage is thick and broad in the middle, in consequence of its lower 

 border being prolonged into a triangular hook-shaped process which curves down- 

 ward and backward between the two bronchi. It terminates on each side in an 

 imperfect ring which encloses the commencement of the bronchi. The cartilage 

 above the last is somewhat broader than the rest at its centre. 



The Fibrous Membrane. The cartilages are enclosed in an elastic fibrous 

 membrane which forms a double layer, one layer, the thicker of the two, passing 

 over the outer surface of the ring, the other over the inner surface ; at the upper 

 and lower margins of the cartilages these two layers blend together to form a single 

 membrane, which connects the rings one with another. They are thus, as it were, 

 imbedded in the membrane. In the space behind, between the extremities of the 

 rings, the membrane forms a single distinct layer. 



The muscular fibres are of the unstriped variety and are disposed in two layers, 

 transverse and longitudinal. 



The transverse fibres (Trachealis muscle, Todd and Bowman), the most inter- 

 nal, form a thin layer which extends transversely between the ends of the carti- 

 lages in the intervals between them at the posterior part of the trachea. Outside 

 of or posterior to these are a few bundles of longitudinal fibres. 



The Mucous Membrane is continuous above with that of the larynx, and below 

 with that of the bronchi. Microscopically, it presents a well-marked basement 

 membrane supporting a layer of columnar ciliated epithelium, between the deeper 

 ends of which are smaller round or elongated cells. It contains a large amount 

 of lymphoid tissue and some tracheal glands. Next to the submucous tissue, the 

 mucous membrane contains elastic fibres, most abundant posteriorly, where they are 

 collected into distinct longitudinal bundles. They are especially numerous about 

 the bifurcation of the trachea. 



The Tracheal Glands (racemose) are found in great abundance at the posterior 

 part of the trachea. They are small, placed upon the outer surface of the fibrous 

 layer ; each is furnished with an excretory duct, which pierces the fibrous and 

 muscular layers and opens on the surface of the mucous membrane. Some glands 

 of smaller size are also found at the sides of the trachea, between the layers of 

 fibrous tissue connecting the rings, and others immediately beneath the mucous 

 coat. The secretion from these glands serves to lubricate the inner surface of the 

 trachea. 



Vessels and Nerves. The trachea is supplied with blood by the inferior thyroid 

 arteries. The veins terminate in the thyroid venous plexus. The nerves are de- 

 rived from the pneumogastric and its recurrent branches and from the sympathetic. 



Surface Form. In the middle line of the neck some of the cartilages of the larynx can be 

 readily distinguished. In the receding angle below the chin the hyoid bone can easily be made 

 out ( see page 230), and a finger's breadth below it is the pomum Adanii, the prominence between 

 the upper borders of the two alae of the thyroid cartilage. About an inch below this, in the 

 middle line, is a depression corresponding to the crico-thyroid space, in which the operation of 

 laryngotomy is performed. This depression is bounded below by a prominent arch, the anterior 

 ring of the cricoid cartilage, below which the trachea can be felt, though it is only in the emaci- 

 ated adult that the separate rings can be distinguished. The lower part of the trachea is not 

 easily made out, for as it descends it is farther removed from the surface. The level of the 

 vocal nords corresponds to the middle of the anterior margin of the thyroid cartilage. 



With the laryngoscope the following structures can be seen : The base of the tongue and 

 the upper surface of the epiglottis, with the glosso-epiglottic folds, the superior aperture of the 

 larynx, bounded on either side by the aryteno-epiglottic folds, in which may be seen two rounded 

 eminences corresponding to the cornicula and cuneiform cartilages. Beneath these, the true 

 and false vocal cords, with the ventricle between them. Still deeper, the cricoid cartilage and 

 some of the anterior parts of the rings of the trachea, and sometimes, in deep inspiration, the 

 bifurcation of the trachea. 



Surgical Anatomy. Foreign bodies often find their way into the air- passages. These may 

 be large substance?, as a piece of meat, which becomes lodged in the upper aperture of the 

 larynx or in the rima glottidis, and cause speedy suffocation unless rapidly got rid of or unless 



