506 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the lungs are entirely passive is shown by the fact that when the thorax is 

 punctured, so as to allow a free communication with the atmosphere, expan- 

 sion of the chest is no longer followed by dilatation of the lungs. During the 

 shrinkage of the thorax the elastic reaction of the lungs plays an active part. 



Respiration, Inspiration, and Expiration. Each respiration or respiratory 

 act consists of an inspiration (enlargement of the thorax and inflation of the 

 lungs) and an expiration (shrinkage of the thorax and the lungs). Accord- 

 ing to some observers, a pause exists after expiration (expiratory pause), but 

 during quiet breathing no such interval can be detected. A pause may be 

 present when the respirations are deep and infrequent. Under certain abnor- 

 mal circumstances a pause may exist between inspiration and expiration 

 (inspiratory pause). 



Inspiration is accomplished by the contraction of certain muscles which are 

 designated inspiratory muscles. Expiration during quiet breathing is essen- 

 tially a passive act, but during forced breathing various muscles are active ; 

 these muscles are distinguished as expiratory muscles. 



During inspiration the thorax is enlarged in the yertical, transverse, and 

 antero-posterior diameters. During quiet breathing the vertical diameter is 

 increased by the descent of the diaphragm, and during deep inspiration it is 

 further increased by the backward and slightly downward movement of the 

 floating ribs, and by the extension of the vertebral column, which raises the 

 sternum with its costal cartilages and ribs. The transverse diameter is in- 

 creased by the elevation and eversion (rotation outward and upward) of the 

 ribs. The antero-posterior diameter is increased by the upward and outward 

 movement of the sternum, costal cartilages, and ribs. During quiet inspiration 

 in men the sternum is not raised to a higher level, but the lower eud is rotated 

 forward and upward. It is only during deep inspiration in the male and in 

 quiet or deep inspiration in women that the sternum as a whole is elevated. 



The movements of the anterior and lateral walls constitute costal respira- 

 tion, and those of the diaphragm diaphragmatic or, as it is sometimes called, 

 abdominal respiration, since the descent of the diaphragm causes protrusion 

 of the abdominal walls. Both types coexist during ordinary respiratory move- 

 ments, but one may be more prominent than the other. The costal type is well 

 marked in women, and the diaphragmatic type in men. These peculiarities 

 are not, however, due to inherent sexual differences, but to dress and heredity. 

 Young children of both sexes exhibit, as a rule, the diaphragmatic type, and 

 it is only near or at puberty that the costal type is developed in the female. 



The chief muscles of inspiration are the diaphragm, the quadrati lumborum, 

 the serrati postid inferiores, the scaleni, the serrati postici superiores, the leva- 

 tores costarum longi et breves, and the intercostales externi et inter cartilaginei. 



Movements of the Diaphragm. The diaphragm is attached by its two 

 crura to the first three or four lumbar vertebrae, to the lower six or seven cos- 

 tal cartilages and adjoining parts of the corresponding ribs, and to the poste- 

 rior surface of the ensiform appendix. It projects into the thoracic cavity in 

 the form of a flattened dome, the highest part being formed by the central 



