The Trachea in man extends from the larynxf to the bifurcation of the tube into the right and left bronchi ; its first superior ring, which is attached to the cricoid cartilage, coincides with the upper border of the body of the fifth cervical vertebra : the point of its bifurcation in the thorax is level with the superior edge of the body of the third dorsal vertebra : it averages in internal diameter from of an inch to 1 inch, and in length from 4 inches to 41 inches4 Variations in the length of the trachea are due to the great range of motion within which the larynx is capable of changing place. The diameter of this tube is greater in the male than in the female, at the lower than the upper ex tremity ; it is nearly cylindrical in figure, and permanently patulous. It is composed in the human subject generally of about eighteen cartilaginous ring,s ; of these rings the pos terior fourth is deficient ; the circle is com pleted at this interval by a musculo-mem branous structure. The tracheal muscle stretches from one extremity of each cartila ginous ring to the other : the jrachea is therefore contitcted in diameter by muscular action, and enlarged by the elasticity of the ring cartilages. The rebounding property of these cartilages results physically from their ring like figure : they tend constantly to straighten themselves; this perpetually acting force preserves the patency of the tube. The con vexity of the tracheal rings is directed for wards, the membranous interval being placed posteriorly : by this arrangement the exemp tion of an important organ from external injury is secured. Against the accident of occlusion during the movements of the neck artful provision is made in the flexible and elastic nature of the structures by which the rings are tied tottether.
The trachea externally is everywhere em braced in loose areolar tissue : upon this circumstance depends the great range of its longitudinal mobility. Its posterior aspect is in contact with the cesophagus, which is interposed between it and the vertebral column. The recurrent laryngeal nerve, a.scending to the larynx, is placed in the in terval between these tubes.
In front of the trachea are situated the sterno-thyroid and sterno-hyoid muscles, which leave an interval in the median line, through which the deep cervical fascia enters to embrace the windpipe. The brachio cephalic and left carotid arteries, leaving the chest through the episternal notch, lie on the trachea near the top of the sternum : above this limit is observed the plexus of the inferior thyroid veins, and near the larynx it is crossed by the isthmus of the thyroid body : on either side and parallel to it are the carotid vessels and the lobes of the th3roid gland. Entering the limits of the thorax, the trachea is in relation anteriorly with the first piece of the sternum, and the sternal extre mities of the sterno-hyoid and thyroid, and to the left, in a descending order, with the in nominate vein, the commencement of the inno minate and left carotid arteries, which tend towards the sides of the tube, with the arch of the aorta and the deep plexus of nerves, and at the point of its bifurcation it is in contact with the pulmonary artery at the place at which this vessel subdivides into branches.
Lying between the two pleurx, the trachea is contained in the posterior mediastinum ; on its right side it is in relation with the pleura and pneumogastric nerve, and on its left, with the carotid artery, the pneumogastric, re current and cardiac nerves.
Structural anatomy of the trachea.— The tra chea is constructed of cartilage, yellow and white fibrous tissue, muscular fibres, blood vessels, lymphatics, and glandules, the whole being internally lined by a dense stratum of ciliated epithelium.
These parts may best be described frorn within outwards.
The tracheal mucous membrane is a develop ment of the pharyngeal. (Ilenle). It forms a layer of 0-024-0.04"' in thickness ; it re solves itself into two distinct layers, including severally two equally distinct orders of cells ; the undermost, resting immediately on the basement membrane, is composed of orbicular and fusiform particles, measuring from 0.004 to 0.005"', and bearing a clear conspicuous nucleus of from 0.0025 to 0.003."' The superficial stratum is constituted of the adult cells ; they consist of club-shaped bodies, armed at the free outermost end with cilia (each single cell carrying about 50 cilia)*, and elongated at the proximal end into a long tapering tail : in length these cells average from 0.015 to 0.02"', and in breadth from 0.0025 to 0.004"', according to the measure ments of K31liker.* This order of ciliated cells is disposed upon a bed of cytoblasts in a double stratum of about 0.006 to 0.01'", in thickness : they differ from ordinary cylinder epithelia in the remarkable length to which the attached extremity is prolonged ; the tail of each cell exhibits quite the character of a yellow filament, and measures from 0.024 to 0.027'" in length.
In the centre of the broad end of each of these cells is contained, without exception, a clear, bright, oblong nucleus, of from 0.003 to 0.0045"/ in length ; and, further, each nucleus bears a very visible nucleolus (e, fig. 201.). The cell contents consist of minute granules and fat molecules. Valentin de scribes a double nucleus in many of these cells.
The cilia attached to the tracheal epithelium are clear, fine continuations of the cell-mem brane : they measure 0.0016 to 0-0022"' in length : each cilium tapers to its free extre mity, that is, it is broader at its base than at its apex. The cilia of the tracheal epithelium are longer and more conspicuous in the em bryo than in the adult : the current excited by their vibration tends in the direction of the laryngeal outlet of the tube. Proofs will be afterwards adduced that these motive or ganules enact no part in the office of re spiration : they subserve a merely mechanical purpose in the process of mucous excretion. These epithelia swell in water, while chromic acid restores them to their original characters (Hannover). By Biermer and Gosselint, the tracheal cilia have been detected in motion 78 hours after death in man. Under normal circumstances no shedding occurs in the epi thelium of the air-passages.