STRUCTURE OF THE LUNGS. 525 



rapid contraction after distention, and by the force with which 

 they expel the air which is used to inflate them when taken out 

 of the thorax. 



The specific gravity of the lungs is not naturally less than 

 that of many other tissues. In a still-born child, sections of it 

 sink in water like a piece of muscle. But when its cells have 

 been once distended by air in respiration it becomes impossible 

 to extrude the air completely (unless the lung is subjected to 

 strong compression) and the lung floats upon the water and 

 appears to have the least specific gravity of all the animal tissues. 

 The lungs are endowed with a considerable degree of elasticity ; 

 this as before observed appears to be derived from the elastic 

 tissue of the bronchia which is spread universally through the 

 lungs. When distended the lungs have a constant tendency 

 either in or out of the body to return upon themselves and 

 expel the air. 



It will now be seen that the proper tissue of the lungs, the 

 parenchyma, or areolar tissue, is very complicated. It consists 

 of the cells of the bronchia for the reception of air, which are 

 formed internally of mucous membrane, and externally most 

 probably of a thin expansion of the yellow elastic ligamentous 

 layer of the bronchia ; of branches of the pulmonary arteries 

 and veins, which run over the outer surface of the cells, the for- 

 mer bringing the black blood, and the latterconveying it away 

 after it has been changed by the action of the air through the 

 walls of the cells ; of the bronchial arteries and veins for the 

 purpose of nutrition ; of absorbent vessels to remove the mole- 

 cules as they become effete ; of filaments of the sympathetic and 

 par vagtim nerves, which preside over the function of hematosis, 

 and put the lungs in connexion with the brain ; and lastly of 

 cellular tissue which unites the whole together. The mucous 

 membrane lining the trachea, bronchia, and air cells, when ex- 

 amined with the microscope, have been found lined with a colum- 

 nar epithelium, mounted with vibratile cilia of the same kind as 

 has been described covering the Schneiderian membrane. The 

 use of the cilia in these organs, it is believed, is that of aiding in 

 urging the secretions upwards towards the larynx. 



