xii MECHANICS OF RESPIRATION 419 



are also involved, opinions are much divided, and no positive facts 

 can be adduced on either side. 



On the other hand, it is clear that in forced respiration the 

 action of all these muscles is reinforced by that of other accessory 

 muscles, whose ordinary office is not to assist the dilatation of the 

 thorax. Such are the sterno-cleido-mastoid, the pectoralis major 

 and minor, the trapezius, serratus and extensors of the vertebral 

 column. Further, those muscles intervene actively which serve to 

 lower the larynx and widen the glottis, as well as the muscles of 

 the palate, fauces, and dilators of the nostrils. 



Another question which is difficult to solve, and as to which 

 opinions are divided, is whether in normal quiet breathing expira- 

 tion takes place passively by simple elastic reaction, or is actively 

 promoted by the interosseous portion of the internal iutercostals, 

 triangularis sterni, and serratus posticus inferior. The great 

 majority of writers, headed by Donders, adopt the first view ; 

 Fick's arguments in favour of the second seem to us, however, to 

 carry a certain weight. He showed that with a little practice, 

 active expiration can be performed voluntarily without throwing 

 the abdominal muscles into any kind of tension. This is easy by 

 concentrating the attention in expiration on dropping the upper 

 ribs and shoulders, and not breathing the air out forcibly, which 

 would throw the abdominal muscles into contraction. If during 

 this intentionally thoracic expiration a water manometer is 

 connected with the oral cavity, the meniscus can be seen to rise 

 4-5 cm., which gives a clear idea of the force exerted by the 

 thoracic muscles, and, according to Fick, proves the expiratory 

 action of the internal intercostals, as to which there has been so 

 much discussion. That these do take part even in normal 

 respiratory rhythm is shown by the fact that the expiratory act 

 can be voluntarily interrupted at any moment, which certainly 

 depends on voluntary inhibition of the expiratory muscles already 

 in action, and not upon the entry of the antagonistic inspiratory 

 muscles, about which we know very Little. 



Another cogent argument in favour of this theory seems to us 

 to lie in the tracings of normal human respiration, recorded by 

 Marey's pneumograph (Fig. 185). Neither the inspiratory nor 

 the expiratory excursions constantly reach the same abscissae, as 

 though drawn between two parallel lines ; but they are now 

 more, now less, extensive, according as in the different breathings 

 the contraction of the antagonistic muscles in individual respira- 

 tions was more or less intense. 



To this it may be added that in dogs (Luciani, 1877) expiration 

 under normal conditions is always active, owing to the intervention 

 of the abdominal muscles, the contraction of which normally 

 prevails over the alternating contractions of the diaphragm, 

 in relation to intra-abdominal pressure, which, as we shall see, 



