120 



RESPIRATION". 



thoracic walls, but are closely applied to them, each covered by a reflection of the serous 

 membrane which lines the cavity of the corresponding side. Thus they necessarily fol- 

 low the movements of expansion and contraction of the thorax. Deep fissures divide 

 the right lung into three lobes and the left lung into two. The surface of the lungs is 

 divided into irregularly-polygonal spaces, from of an inch to an inch in diameter, 

 which mark what are sometimes called the pulmonary lobules; although this term is in- 

 correct, 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, the smallest, 

 which are from T |-o to T V of an inch in diameter, open into a collection of oblong vesicles, 



which are the air-cells. Each collection 

 of vesicles constitutes one of the true pul- 

 monary lobules and is from T^ to -^ of 

 an inch in diameter. After entering the 

 lobule, the tube forms a sort of tortuous 

 central canal, sending off branches which 

 terminate in groups of from eight to fif- 

 teen 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 circular constrictions, or rugae. In 

 the healthy lung of the adult, after death, 

 they measure from ^^ to T 7 or -fa of an 

 inch 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 surface. 

 There are considerable variations 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 numerous small elastic fibres, 

 which do not form distinct bundles for 

 each air-cell, but anastomose 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 im- 

 portant part in expelling 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 scales of pavement-epithelium, from ^Vo to ^-^ of 

 an inch in diameter, which are applied directly to the walls of the blood-vessels. The 

 epithelium here does not seem to be regularly desquamated, as in other situations. Ex- 

 amination of injected specimens shows that the blood-vessels are so situated between the 

 cells, that the blood in the greater part of their circumference is exposed to the action of 

 the air. 



The entire mass of venous blood is distributed in the lungs by the pulmonary artery. 

 Arterial blood is conveyed to these organs by the bronchial arteries, which ramify and 

 subdivide on the bronchial tubes and follow their course into the lungs, for the nourish- 



FIG. 38. Mould of a terminal bronchus and a growp of 

 air-cells moderately distended by injection, from, the 

 human subject, (Robin.) 



