RESPIRATION 213 



intercartilaginei and the resistance of the sternum to further 

 displacement exerted on the cartilages. The whole arrange- 

 ment is perfectly adapted to permit the expansion of the roughly 

 conical upper lobes of the lungs. 



(3) The lower costal series, consisting of the ribs from the 

 sixth to the tenth. These ribs, with their muscles, form a 

 mechanism which normally acts along with the diaphragm 

 (Keith). They are so arranged that in inspiration the lateral 

 and anterior part of each moves outwards to a greater extent 

 than the one above it. There is not only a rotation around 

 a transverse axis, by which the lower end of the sternum, 

 connected to these ribs by the combined cartilages of the sixth 

 to the ninth, is elevated, but also a rotation around an antero- 

 posterior axis. The movement of the lower ribs results, there- 

 fore, in increasing both the back-to-front diameter and the 

 transverse diameter of the lower portion of the thorax. The 

 widening of the thorax from side to side may also be in a slight 

 degree ascribed to a twisting movement of the ribs, which tends 

 to evert their lower borders. With the diaphragm, these lower 

 ribs arranged in a vertical series of not very different curvature 

 constitute a mechanism for the inspiratory expansion of the 

 roughly cylindrical lower lobes of the lungs. 



Expiration in perfectly tranquil breathing is brought about 

 with less aid from active muscular contraction. The sense of 

 effort disappears as soon as the chest ceases to expand. The 

 diaphragm and the elevators of the ribs relax. The structures 

 that have been stretched or twisted recoil into their original 

 positions ; the structures that have been raised against the 

 force of gravity fall back by their weight, and in the measure 

 in which the pressure increases in the thoracic cavity the elasticity 

 of the lungs causes them to shrink. The pressure in the alveoli, 

 which at the end of inspiration was just equal to that of the 

 atmosphere, is thus increased, and the air expelled. It is probable 

 that, even in man and in quiet respiration, the interosseous 

 portions of the internal intercostals help by their contraction in 

 depressing the ribs, and that a slight contraction of the abdominal 

 muscles hastens the return of the diaphragm to its position of rest. 

 In reptiles and birds, expiration is normally effected by an active 

 muscular contraction. This is also true in some mammals the 

 rabbit, for instance, in which the external oblique muscles of the 

 abdominal wall take an important share in the expiratory act. 



Types of Respiration. Differences exist also, not only between 

 different groups of animals, but even between women and men, 

 in the relative importance in inspiration of the diaphragm and 

 the muscles that raise the lower ribs on the one hand, and the 

 muscles that elevate the upper ribs on the other. When the 



