236 FUNCTIONS OF NUTRITION. 



lungs their property of resiliency. The pavement epithelium lining 

 the ultimate bronchial tubes extends into the lobules and vesicles, 

 forming, according to Kolliker, a continuous investment of their in- 

 ternal surface. 



The extensive involution of the respiratory membrane, resulting 

 from the multiplication of the bronchial tubes and vesicles, in the 

 lungs of man and mammalians, increases in a high degree the activity 

 of respiration ; since the blood in the capillary vessels, distributed in 

 thin layers over so large a surface, in immediate proximity to the air 

 in the pulmonary cavities, is placed under the most favorable conditions 

 for rapid arterialization. 



Movements of Respiration, 



The air in the pulmonary lobules and vesicles, being used for the 

 arterialization of the blood, is rapidly altered in composition, and 

 requires to be replaced by a fresh supply. Its renewal is effected by 

 alternate movements of expansion and collapse of the chest, following 

 each other in regular succession, known respectively as the " movement 

 of inspiration," and the " movement of expiration." 



Movement of Inspiration. The expansion of the chest is produced 

 by two sets of muscles, namely, the diaphragm and the intercostals. 

 The diaphragm is a vaulted muscular sheet, forming the floor of the 

 thorax, its edges being attached to the lower extremity of the sternum, 

 the inferior costal cartilages, the borders of the lower ribs, and the 

 bodies of the lumbar vertebrae, whence its fibres run upward and 

 inward, to the triangular tendinous expansion at its centre. In the 

 relaxed condition, its convexity rises into the chest, as high as the level 

 of the fifth rib. When its muscular fibres contract, they draw its cen- 

 tral tendon downward, depressing the abdominal organs, and enlarging 

 the cavity of the chest in a vertical direction. At the same time, by 

 the contraction of the intercostal muscles, the ribs are drawn upward 

 and outward, rotating upon their articulations with the spinal column, 

 and expanding the chest from side to side. The sternum also rises 

 slightly and increases to some extent the antero-posterior diameter of 

 the thorax. By these changes, the cavity of the lungs is enlarged in 

 every direction, and the air penetrates, by the force of aspiration, 

 through the trachea and bronchial tubes, to the pulmonary lobules and 

 vesicles. 



The action of the respiratory muscles is indicated externally by two 

 different motions, namely, the expansion of the chest, due to the inter- 

 costals, and the protrusion of the abdomen, caused by the descent of 

 the diaphragm. In children, as well as in the adult male, under ordi- 

 nary conditions, the diaphragm performs most of the work, and the 

 movements of the abdomen are the only ones especially noticeable. 

 Any unusual exertion, however, produces an increased expansion of the 

 chest ; and the movement of the ribs becomes more plainly visible after 



