114 



RESPIRATION RESPIRATORY MOVEMENTS. 



vided into irregularly polygonal spaces, of an inch to an inch (6-4 to 25*4 

 mm.) in diameter, which mark what are sometimes called the pulmonary 

 lobules ; although this term is incorrect, as each of these divisions includes 

 quite a number of the true lobules. 



Following out the bronchial tubes from the diameter of -fa of an inch 

 (0-5 mm.), the smallest, which are T fg- to ^ O f an inch (0*21 to 0-33 mm.) 

 in diameter, open into a collection of oblong vesicles, which are the air- 

 cells. Each collection of vesicles constitutes one of the true pulmonary 

 lobules and is -^ to ^ of an inch (0-5 to 2-1 mm.) in diameter. After 

 entering the lobule, the tube forms a tortuous central canal, sending off 

 branches which terminate in groups of eight to fifteen pulmonary cells. 

 The cells are a little deeper than they are wide and have each a rounded, 



blind extremity. Some are smooth, 

 but many are marked by little cir- 

 cular constrictions, or rugae. In 

 the healthy lung of the adult, after 

 death, they measure -g-^-g- to T J 7 or 

 TV of an inch (0-125 to 0-21 or 

 0-36 mm.) in diameter, but are 

 capable of very great distention. 

 The smallest cells are in the deep 

 portions of the lungs, and the 

 largest are situated near the sur- 

 face. There are considerable vari- 

 ations in the size of the cells at 

 different periods of life. The 

 smallest cells are found in young 

 children, and they progressively 

 increase in size with age. The 

 walls of the air-cells contain very 

 abundant small, elastic fibres, 

 which do not form distinct bun- 

 dles for each air-cell, but anasto- 

 mose freely with each other, so 

 that the same fibres belong to two 

 or more cells. This structure is 

 peculiar to the parenchyma of the lungs and gives to these organs their great 

 distensibility and elasticity, properties which play an important part in ex- 

 pelling the air from the chest, as a consequence simply of cessation of the 

 action of the inspiratory muscles. Interwoven with these elastic fibres, is the 

 richest plexus of capillary blood-vessels found in the economy. The vessels 

 are larger than the capillaries in other situations, and the plexus is so close 

 that the spaces between them are narrower than the vessels themselves. 

 When distended, the blood-vessels form the greatest part of the walls of the 

 cells. 



Lining the air-cells, are very thin cells of flattened epithelium, 



FIG. 42. Mould of a terminal bronchus and a group of 

 air-cells moderately distended by injection, from 

 the human subject (Robin). 



