404 PHYSIOLOGY CHAP. 



Each infundibulum is surrounded by connective tissue, rich in 

 elastic fibres, and containing blood and lymph vessels. A certain 

 number of infundibuli, with their respective bronchioles, inter- 

 connected by connective and elastic tissue, and by larger 

 vessels, make up the pulmonary lobules, provided in their turn 

 with lobular bronchi, which arise from the junction of the 

 bronchioles. A number of these lobules, united by the said 

 tissues, by still larger vessels, and by lobar bronchi, form the 

 pulmonary lobes which in conjunction make the lung. 



The structure of the respiratory passages changes gradually 

 from bronchioles to lobular bronchi, from these to the lobar 

 bronchi, from the larger bronchi to the trachea. We must confine 

 ourselves to stating that the bronchioles are lined with a single 

 layer of cubical, non-ciliated, epithelial cells, surrounded with a 

 thin sheath of connective and elastic tissue, sparsely provided 

 with smooth muscle fibres. The epithelium of the lobular bronchi 

 is cylindrical and ciliated, and the tube consists of an external 

 sheath of connective tissue, rich in elastic fibres and concentrically 

 arranged smooth muscle cells. In the interlobular and lobular 

 bronchi the tube is more muscular and somewhat rigid, because 

 the coat of elastic and connective tissue, found on the outside of 

 the circular muscular layer, contains small irregularly distributed 

 plates of hyaline cartilage. These cartilage plates become larger 

 in the direction from medium to greater bronchi, while the 

 transverse layer of muscle diminishes. The mucosa thickens 

 from small to large bronchi. The epithelium is stratified (three 

 or more layers of cells); between the cylindrical ciliated cells 

 mucous goblet cells are occasionally visible. Beneath the epithelium 

 is a reticulated adenoid tissue with thin elastic fibres, and on the 

 outside a compact and conspicuous elastic layer, formed of the 

 predominantly longitudinal fibres, which are arranged in a 

 network, in which are many small mucoid and acinous glands, 

 opening into the lumen of the bronchus by small ducts that 

 pierce the elastic layer, reticulated tissue, and epithelium. The 

 mucous secretion of the goblet cells and small glands intercepts 

 the solid particles introduced with the air, while the cilia drive 

 the mucus with the agglutinated particles towards the trachea 

 and larynx. 



The tracheal tube is much more rigid, since the elastic sheath 

 is more external and tougher, and contains rings of hyaline 

 cartilage, interrupted at the back and completed by bundles of 

 smooth muscle fibres, stretched transversely across the ends of 

 the cartilage, while a few other muscle bundles run longitudinally 

 outside the former. The tracheal mucosa is not essentially 

 different from that of the greater bronchi, and contains more 

 mucous glands. The strong elastic layer adjusts the mucosa to 

 the longitudinal alterations of the tube, keeping it tense and 



