262 



RESPIRATION, ORGANS OF. 



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 O'S'" in thickness. The ele- 

 ments of each fascicle are O'OS'" long, and from 

 0-002 to O'OOl'" 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 advantage 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 umtriped 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 (/, 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. 



The meshes of the web are considerably 



larger tlian those of the true pulmonary 

 plexus, A network of lymphatics also exists 

 in the submucous tissue of the trachea. The 

 meshes of this web are larger and more 

 round-angled than those formed by the blood- 

 vessels. 



The Bronchi are the primary divisions of 

 the trachea. They consist of two single tubes, 

 one on either side, leading to either lung. 

 Immediately on entering the substance of the 

 lung, they undergo divisions into as many 

 branches as there are lobes. The bronchus on 

 both sides is posterior to the vessels at the 

 root of the lung, and is surrounded by the 

 bronchial vessels and glands and by the pul- 

 monary nerves. The right bronchus presents 

 a greater diameter than the left ; it is shorter 

 than the latter, and passes almost horizon- 

 tally outwards above the right pulmonary 

 artery, and the vena azygos arches over its 

 upper part. The left bronchus, nearly double 

 the length of the right, passes obliquely down- 

 wards through the arch of the aorta, and, in 

 its extension to the lung, lies on the oeso- 

 phagus, thoracic duct, and descending aorta, 

 being below the level of the pulmonary artery. 

 The right and left bronchi, before they pene- 

 trate the pulmonary substance, exhibit the 

 same structure as the trachea. The cartila- 

 ginous rings are imperfect behind, like those 

 of the trachea ; the interval being completed 

 by a musrulo-membranous structure. These 

 cartilages on the right bronchus vary from six 

 to eight in number, being shorter and narrower 

 than those of the trachea. Those of the left 

 amount to from nine to twelve. The mem- 

 branous posterior space is thickly interspersed 

 with glands like those of the trachea. After 

 entering into the substance of the lungs, the 

 bronchial tubes become modified in structure. 

 The cartilages are no longer parallelly dis- 

 posed semi-rings. They are arranged in 

 irregular curved pieces over ever)' point of 

 the circumference of the tubes. They do not 

 exist in the corresponding part of the bird's 

 lungs. Thus the bronchi may be classified 

 as the extra and infra pulmonary.* Com- 

 pressed at every point of their circumference 

 by the surrounding pulmonary substance, it 

 is evident that the tubes within the lungs 

 should be constructed such as to be capable 

 of resisting external pressure. This pro- 

 vision is adroitly realised in the manner in 

 which the cartilaginous pieces are disposed 

 over the walls of the tubes. Each piece of 

 cartilage, having a main circular arrangement, 

 is a small segment of a large circle. They 

 are tied together by muscular and elastic 

 fibres. Thus they form a patch-work, cylin- 

 drical frame. The diameter of the tubes is 

 diminished by the approximation of the sepa- 

 rate pieces under muscular agency. The 

 patency of these tubes, unlike that of those of 

 which the walls are exclusively membranous, 



* By some authors the air-tubes within the lungs 

 are distinguished as the bronchia anil bronchiola, 

 after Haller, who thus alludes to them : " Eos 

 ramos Veteres bronchia syringes et aortas dix- 

 eraut," Elem. Phvs. 



