534 
MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
mutual action of the scaleni and first rib on each other; while they raise the rib, the 
rib moves outwards and carries with it the scaleni ; the scaleni then widen the space 
into which the lung rises and which they bound. 
84. Pleural scalenus *. 
In addition to the ordinary scaleni, there is usually a special scalenus to expand 
the summit of each lung. This pleural scalenus arises from the transverse process 
of the seventh cervical vertebra, becomes tendinous and aponeurotic, and is inserted 
by a funnel-like tendinous web into the whole circuit of the first rib. The pleural 
scalenus somewhat resembles the large funnel-like scalenus of the Porpoise, that 
enlarges the lung in the neck, § 71 - 
85. Intercostal muscles , Plate XXIX. figs. XIII. a.b., XIV. a. b., 'X.V.a.b. 
§§ 46.48.49.60.61. 
The action of each intercostal muscle depends entirely on the movements of the 
rib into which it is inserted; as. the thoracic set of ribs approach each other in 
front, while the diaphragmatic and intermediate sets recede from each other, we 
might have anticipated that the intercostal muscles between the former have different 
respiratory actions from those between the latter. We have already seen, in the 
lower Mammalia, how completely the actions of the intercostal muscles correspond 
with the motions of the ribs 48. 49. 60. 61.) ; we shall find the same correspond- 
ence in Man. 
The external intercostals of the seven superior ribs (all the thoracic and two of 
the intermediate sets) are throughout inspiratory ; they raise these ribs, make them 
to glide on each other towards the vertebrae (§§ 6.7), and push backwards, through the 
ribs, the vertebral column ; those of the thoracic set, in addition, make the oblique 
inner surfaces of the ribs more nearly horizontal, as diagram R illustrates. 
Diagram R. 
The anterior or intercartilaginous portions of the superior five internal intercostals 
are all inspiratory (fig. XIII. b) ; they raise the costal cartilages and make them glide 
on each other towards the sternum ; the four superior internal intercostal muscles 
are inspiratory anteriorly, where the ribs approach each other, and expiratory poste- 
* Fig. XXVI. Archives of the Royal Society. 
