260 MANUAL OF HISTOLOGY. 



dichotomous manner, diminishing gradually in calibre, the 

 combined area of the branches, however, always exceeding that 

 of the trunk from which they' spring. No change occurs in 

 their structure, except a gradual thinning of their walls, until 

 they reach a diameter of about 1 mm., when the cartilages 

 disappear and the attenuation is more marked. The circular 

 muscular fibres continue to exist, as also the longitudinal elas- 

 tic fibres, but the mucous glands disappear. After a still 

 further division the tubes are diminished to a diameter of .20 

 to .30 mm., the muscular fibres become more sparse, and the 

 epithelium is reduced to a single layer of low, somewhat cubi- 

 cal cells, which are still ciliated. These are the lobular bronchi, 

 each one going to a single pulmonary lobule. The lobular 

 bronchi each give off ten to fifteen smaller tubes, known as the 

 terminal bronchi or bronchioles. They are straight and cylin- 

 drical, their walls are very thin and 

 delicate, and their epithelial cells 

 gradually lose their cilia and become 

 flattened plates. Each bronchiole 

 leads to a smaller division of the lob- 

 ule, called an acinus or lobulette. 1 The 

 bronchioles divide into short canals, 

 the alveolar passages, usually three 



Fio. 110. A system of alveolar pas- -i mu 11 j.u* 



ges with infuridibuii from an ape's for each acinus. Their walls are thin 



Inng : a, terminal bronchial twig ; 6. 6, 111 -i 1 1 ' i _e 



intnndibuia; c, c, alveolar passages, and bulge outward on all sides, torm- 



Magnifled 10 times. F. E. Schulze. . ,, , . . . , .. 



ing, externally, little projections or 



elevations ; internally, shallow depressions or cavities which 

 open into the calibre of the tube. They also give off secondary 

 branches, called infundibula, which have groups of such little 

 cavities attached to, and opening into them. The little cavities 

 are the alveoli or air-cells of the lung. From this description 

 it will be seen that each lobule has ten to fifteen acini or lobu- 

 lettes, and that the lobulette is made up of alveoli or air-cells, 

 which open into common spaces or infundibula, which in turn 

 communicate with the alveolar passages. The alveoli, which 

 are connected with the infundibula, are called terminal alve- 

 oli ; those which open on the sides of the alveolar passage are 

 called the parietal alveoli. The latter are called, by Dr. Wa- 

 ters, the broncJiial alveoli. The alveolar passages, infundibula, 



1 Dr. Waters : The Anatomy of the Human Lung, London, I860. 



