RESPIRATION, ORGANS OF. 



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 rings ; 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.trachea is therefore 

 contracted 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 together. 



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 oesophagus, which is 

 interposed between it and the vertebral 

 column. The recurrent laryngeal nerve, 

 ascending 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 thyroid 

 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 pleurae, 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. 



259 



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 from 

 within outwards. 



The tracheal mucous membrane is a develop- 

 ment of the pharyngeal. (Henle). It forms a 



Fig. 204. 



Vertical cutting through the epithelial and sub-epi- 

 thelial layer of the trachea. (After KoUiker.) 

 a, b, basement or homogeneous membrane ; c, the 

 first race or growth of epithelial cells ; c, d, the last, 

 further evolved ; e, the adult, surface, ciliated cells. 



layer of 0-024 OW 7 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 



Fig. 205. 



Separate cells taken from, the epithelium of the trachea, 

 the lowest, smallest, and globular, being the youngest; 

 the uppermost, elongated, and ciliated, the oldest. 



* According to Valentin's counting, each cell 

 supports no more than from ten to twenty-two 

 cilia ; but I have often reckoned many more. 



2 



