MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 229 



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 con- 

 stitute 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 struc- 

 tures 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 respira- 

 tion, the interosseous portions of the internal intercostals help by 

 their contraction in depressing the ribs, and that a slight contrac- 

 tion 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 movements of 

 the diaphragm predominate, the respiration is said to be of the 

 abdominal or diaphragmatic type ; when the movements of the upper 

 ribs and sternum are most conspicuous, of the costal or thoracic type. 

 In abdominal respiration, the inspiratory movement commences at 

 the diaphragm, and then involves the lower ribs and the tip of the 

 sternum. In costal respiration, the upper ribs initiate the move- 

 ment, and are followed by the abdomen. In the rabbit, during 

 quiet breathing, the respiration is purely diaphragmatic, the ribs 

 remain motionless; and herbivorous animals in general conform 

 more or less closely to this type. In the carnivora, on the contrary, 

 the costal type prevails. Man allies himself as regards his respira- 

 tion with the rabbit and the sheep ; he uses his diaphragm more than 

 his upper ribs. Civilized woman falls into the class of the wolf and 

 the tiger; she uses her upper ribs more than her diaphragm. The 

 cause of the difference between men and women has been much 

 discussed. It is not a primitive sexual difference, for it is far from 



