RESPIRATION. 143 



number of distinct lobules (the pulmonary lobules'), connected by 

 interlobular connective tissue. These lobules vary in size, are of 

 an oblong shape, and are composed of the ultimate ramifications of 

 the bronchial tubes, within which are contained the air-vesicles or. 

 cells. The walls of the air-vesicles, exceedingly thin and delicate, 

 are lined internally by a layer of tessellated epithelium, externally 

 covered by elastic fibers, which give the lungs their elasticity and 

 distensibility. 



The venous blood is distributed to the lungs for aeration by the 

 pulmonary artery, the terminal branches of which form a rich plexus 

 of capillary vessels surrounding the air-cells ; the air and blood are 

 thus brought into intimate relationship, being separated only by the 

 delicate walls of the air-cells and capillaries. 



The thoracic cavity, in which the respiratory organs are lodged, 

 is of a conic shape, having its apex directed upward, its base down- 

 ward. Its framework is formed posteriorly by the spinal column, 

 anteriorly by the sternum, and laterally by the ribs and costal car- 

 tilages. Between and over the ribs lie muscles, fascia, and skin, 

 above, the thorax is completely closed by the structures passing 

 into it and by the cervical fascia and skin ; below, it is closed by 

 the diaphragm. It is, therefore, an air-tight cavity. 



The Pleura. Each lung is surrounded by a closed serous mem- 

 brane (the pleura), one layer of which (the visceral} is reflected 

 over the lung; the other (the parietal}, reflected over the wall of 

 the thorax ; between the two layers is a small amount of fluid, which 

 prevents friction during the play of the lungs in respiration. 



Owing to the elastic tissue which is present in the lungs, they are 

 very readily distensible ; so much so, indeed, that the pressure of 

 the air inside the trachea and lungs is sufficient to distend them 

 until they completely fill all parts of the thoracic cavity not occupied 

 by the heart and great vessels. The elastic tissue endows them not 

 only with distensibility, but also with the power of elastic recoil, 

 by which they are enabled to accommodate themselves to all varia- 

 tions in the size of the thoracic cavity. 



When the chest-walls recede, the air within the lungs expands and 

 presses them against the ribs ; when the chest-walls contract, the air 

 being driven out, the elastic tissue recoils and the lungs return to 

 their original condition. The movements of the lungs are, there- 

 fore, entirely passive. 



