The meshes of the web are considerably larger than 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 ceso 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 musculo-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 every 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, is not the result of atmospheric pressure, nor of the action of the thoracic inspiratory mus cles : it is due to the elastic property of the cartilaginous pieces. A " membranous inter val " in the walls of the intra-pulmonary bronchi would obviously expose the air-pas sages to injurious pressure. The-rnuscular fibres which always belong to the unstriped kind in this class of bronchial tube are ar ranged chiefly circularly., but partly longi tudinally. They are thus enabled, not only to contract the calibre, but to diminish the length, acting, therefore, as important expiratory powers. As the dissector advances towards the branches of the " bronchial tree," the cartila ginous fragments in the walls of the tubes be come thinner and smaller, and more and more distantly placed, until eventually they cease altogether, the walls of the tubes being com posed of nothing but fibro-membrane. Here the tubes are maintained in the open state in part by the expansive force of the contained column of air. The collapse of these pas sages leads to the atalectasis of that portion of the lung to which they lead. At this point the gravest impediment to respiration, in bronchitis, occurs ; here also is chiefly seated the obstacle to expiration in some forms of emphysema ; and these musculo membranous tubules are the scene of spasm during the paroxysms of spasmodic asthma. The parietes of the bronchial tubes within the substance of the lungs bear glands like those of the trachea, and, like the latter, they are lined internally by a ciliated cylinder epithelium. This epithelium terminates, by abruptly-defined borders, at the commence ment ofthe true pulmonary tissue. (a) This fact was first shown by Mr. Rainey*: subsequent observers have, however, proved, contrary to the opinion of Rainey, that the epithelial layer does not wholly cease at:these points (b). lt is prolonged over the respiratory segment under the character of pavement-epithelium.