The tracheal cartilaginous rings come next to be described. Each cartilage forms a little more than three quarters of a circle ( e, fig. 207.). It embraces the anterior three fourths of the tracheal tube. The deficient portion, comprising the remaining fourth of the circle, is completed by muscle and mem brane and elastic tissue (k, fig,. 207.). This description applies only to the case of the human subject. In the sheep the posterior ends of the tracheal cartilages meet, to pro ject behind as spinous processes, thus con cealing the trachealis muscle. In the horse the same parts of the ring cartilages over lap. This arrangement prevails also in the dog. In the ox the posterior ends of the tracheal rings are everted. Each cartilage is embraced in a fibrous perichondrium (f, fig. 207.) which is intimately united to the inter-annular fibrous tissue.
In ultimate structure these cartilages are chiefly composed of cells interspersed through a fibrous basis (b, c, d, c, b, fig. 206.). The cells are largest and doubly nucleated in the centre of each cartilage (d). At the inner and outer surface these cells become flat tened and elongated (b, b). In different parts of' a vertical section of the cartilage they present different directions as regards their long axes (d). They are charged, in addition to the nucleus, with minute fat molecules. The first and last rings of the trachea are figured differently from the rest. The first constitutes a complete ring of cartilage, and is frequently anchylosed to the cricoid. The last is prolonged backwards over the mem branous interval ; consisting of more than one piece, it arches over the angle where the trachea bifurcates into the primary bronchi. By such skilful adaptation of these elastic rings, the occlusion of the air-tube is prevented during the varied motions of the neck and chest. Unlike the laryngeal cartilages, the tracheal manifest no disposi tion to ossify. They amount, in man, to from 16 to 20 in number, and measure each in width, from above downwards, about one line and a half. They frequently fork at their posterior extremities, and sometimes two or more be come united. The internal aspect of each ring is thickened into a rounded form. This dis position of material exhibits an advantage ous mechanical adaptation to the chief purpose which these cartilages are designed to fulfil. It confers upon them a powerful straightening tendency. To this continuously-acting me chanical force the trachea owes its patency. If the membranous interval of the tube be sud denly cut longitudinally, the rings will rebound almost into straight lines.
The tracheal muscles (k, fig. 207.) extend transversely between the free ends of the ring-cartilages, and also in part between these rings in form of oblique fasciculi, a disposition which enables them to influence both the transverse and longitudinal movements of the tube. The fibres of each muscle do not extend in a regularly parallel direction from one ex tremity of the ring-cartilage to the other. They interweave frequently and irregularly. They are also intermixed with elastic tissue. They form a layer of 0.3"/ in thickness. The ele ments of each fascicle are 0.03"/ long, and from 0-002 to 0.004'" broad. Along the course of the fibres, at long intervals, are observed nu clei of very elongated figure.
The tendon of each muscle is attached to the internal surface of the ring cartilage, at some distance inwards from its extremities. This arrangement confers upon the muscle great mechanical advantag,e in the act of nar rowing the tube. In the horse, this disposi tion of the tendons is still more marked. In man and mammalia the tracheal muscles be long to the unstriped variety. In birds they are striated : this is also the case in the muscles of the snake. The layer formed by these muscles is perforated by the ducts of the tracheal glands, which are thickly dis tributed over its posterior surface in form of reddish granules (l, fig. 207.).
The musculo-membranous portion of the trachea in man is narrower at the laryngeal end of the tube than at the thoracic. Above, less than one third of the circumference of the cylinder is membranous ; below, more, except at the actual point of bifurcation. The tra cheal glands are more numerous at the tho racic than at the laryngeal end. At the former point, both the vascularity and sen sibility of the tube are greater than at the latter. It is the innermost and immediate inlet into the organs of breathing. Here, in man and mammalia, the excitability and high organisation of the larynx are repeated, and in birds a second larynx is developed. The presence of mucus at this point excites immediate cough. The trachea is to the lungs as the apex of a rapidly-expanding cone is to its base.
The arteries (i,fig. 207.) of the trachea are chiefly derived from the inferior thyroid, and the nerves from the sympathetic and the recur rent of the pneumogastric. .By injection it is easy to demonstrate the existence of a sub mucous capillary plexus.