162 MOVEMENTS OF EESPIEATIOX. 



also from front to back. In men, this effect* takes place more particular- 

 ly through the movements of the lower ribs, and this form of respiration 

 is therefore sometimes called the inferior-costal ; but in women the upper 

 jibs are more movable, the dilatation of the chest is there greater, and 

 the respiration therefore designated as the superior -costal. In these 

 movements of the ribs, and especially in violent respiration, many mus- 

 cles are involved. 



In the reverse act, that is, in expiration, or the expulsion of air through 

 the trachea, the floor of the chest is raised. The diaphragm, when it 

 contracted, made pressure upon the viscera of the abdomen, and forced 

 the muscular walls of that cavity outward ; but, as soon as the diaphragm 

 relaxes, the abdominal muscles contract, and thus an antagonizing force 

 is originated which tends to expel the air. In this the elasticity of the 

 lungs and of the walls of the thorax itself affords a great assistance. 

 Owing to Jhis elasticity, the muscular exertion required for the introduc- 

 tion of the air greatly exceeds that required for its expulsion. 



In tranquil respiration, we may regard the changing of the air to be 

 accomplished by the alternate depression and elevation of the diaphrag- 

 matic floor of the chest. On an average, this takes place 1 7 times in a 

 minute, and in an adult of the standard size we may assume that 17 

 cubic inches of air are introduced at each inspiration. Of every five 

 breaths one is usually deeper than the other four. The statement often 

 made, that five pulsations correspond to one respiration, must be received 

 with a certain restriction. In pneumonia, the respirations may be to 

 the pulsations as 1 to 2 ; in typhoid fevers, as 1 to 8 ; and even in a 

 state of health there may be considerable variations. 



By muscular movements, which thus call into action atmospheric pres- 

 sure, the air is drawn, but not forced, into the respiratory apparatus. 

 Considering, however, the solid contents of the lungs, which can not be 

 taken at less than 200 cubic inches, it is clear that the amount is not 

 more than sufficient to fill the nasal passages, the trachea, and the larger 

 ramifications of the bronchial tubes. Lying nearest to the outlet, it 

 would be the first to be expelled by the act of expiration. There could 

 be no exchange of the fresh for the foul air, unless some additional means 

 were employed for accomplishing its transference from the larger ramifi- 

 cations of the bronchial tubes to the remotest air-cells. 



2d. The transference of fresh air to the cells is accomplished by re- 

 sorting to two different principles, the diffusion of free gases into one an- 

 other, and muscular contraction. 



An estimate of the relative share which each of these takes is arrived 

 Effect of gase- at by an examination of the absolute velocity with which 

 Us^oforganic S ases diffuse into one another. The statement that gases 

 muscle fibres, act as vacua to each other has led to some very erroneous 



