322 LABOURED INSPIRATION. [BOOK n. 



function, those parts which lie between the osseous ribs act as 

 depressors, i.e. as expiratory in function. 



In the well-known model invented by Bernoulli and adopted 

 by Hamberger, consisting of two rigid bars, representing the ribs, 

 moving vertically by means of their articulations with an upright 

 representing the spine and connected at their free ends by a piece 

 representing the sternum, it is undoubtedly true that stretched 

 elastic bands attached to the bars in such a way as to represent 

 respectively the external and internal intercostals, viz. sloping in 

 the one case downwards and forwards and in the other downwards 

 and backwards, do, on being left free to contract, in the former 

 case elevate and in the latter depress the ribs. Such a model 

 however does not fairly represent the natural conditions of the 

 ribs, which are not straight and rigid, but peculiarly curved and of 

 varying elasticity, capable moreover of rotation on their own axes, 

 and having their movements determined by the characters of 

 their vertebral articulations. The mechanical conditions in fact 

 of these muscles are so complex, that a deduction of their actions 

 from simple mechanical principles, or from the direction of the 

 fibres, must be exceedingly difficult and dangerous. Actual experi- 

 ments on the cat and dog tend to shew that in these animals the 

 contraction of the internal intercostals, along their whole length, 

 takes place, in point of time, alternately with that of the 

 diaphragm, and thus afford an argument in favour of these muscles 

 being expiratory in function. 



Next in importance to the external intercostals come the 

 levatores costarum, which, though small muscles, are able, from 

 the nearness of their costal insertions to the fulcrum, to produce 

 considerable movement of the sternal ends of the ribs. The 

 external intercostals and the levatores costarum with the scaleni 

 may fairly be said to be the elevators of the ribs, i.e. the chief 

 muscles of costal inspiration in normal breathing. 



Additional space in the transverse diameter is afforded probably 

 by the rotation of the ribs on an antero-posterior axis; but this 

 movement is quite subsidiary and unimportant. When the chest 

 is at rest, the ribs are somewhat inclined with their lower borders 

 directed inwards as well as downwards. When they are drawn up 

 by the action of the intercostal muscles, their lower borders are 

 everted. Thus their flat sides are presented to the thoracic cavity, 

 which is thereby slightly increased in width. 



Laboured Inspiration. When respiration becomes laboured, 

 other muscles are brought into play. The scaleni are strongly 

 contracted, so as to raise or at least give a very fixed support 

 to the first and second ribs. In the same way the serratus 

 posticus superior, which descends from the fixed spine in the lower 

 cervical and upper dorsal regions to the second, third, fourth, and 

 fifth ribs, by its contractions raises those ribs. In laboured breath- 



