126 Mr. Le Gros Clark on the Mechanism of Respiration. 



ments. The deeply notched form of the chest below, with its movable 

 elastic boundary of cartilage, is well adapted to admit of these necessary 

 movements of alternate expansion and contraction. 



14. A difference of opinion exists as to the action of the intercostal 

 muscles, some physiologists assuming that the external and internal sets of 

 muscles act independently of each other and as antagonists ; others sup- 

 posing that different parts of the same muscle perform diverse functions. 

 I am disposed to believe that both these conjectures are incorrect ; and that 

 Haller is right in his opinion, that both sets of intercostals act as muscles 

 of inspiration. They act, in concert with the scaleni, in drawing the ribs 

 upwards ; they also approximate them, and rotate them on their axes, — a 

 result which is facilitated by the increasing mobility of each pair of ribs as we 

 descend from the first to the last. The effect of such action is to afford 

 a fixed circumference from which the horizontal portion of the diaphragm 

 can act without drawing the ribs inwards ; at the same time that the 

 general capacity of the chest is augmented, though its vertical diameter, 

 so far as the intercostal action is concerned, is shortened ; and the crura of 

 the diaphragm must also aid importantly in steadying and fixing the central 

 tendon during inspiration, and in preserving the pericardium from that 

 encroachment to which it would be liable if the central tendon were not 

 thus fixed at its back part, and drawn downwards from the chest. 



15. But the action of the intercostal muscles, which has been a subject 

 of so much dispute, will repay a more careful examination. 



The posterior portion of the external intercostals, reaching from the 

 angles to the tubercles of the ribs, unaccompanied by the internal inter- 

 costals, has an action similar to that of the levatores costarum, the upper 

 of the two ribs to w r hich each fibre is attached forming a fixed point from 

 which the lower rib may be influenced in direction. The anterior part of 

 the internal intercostals, passing between the costal cartilages unaccom- 

 panied by the external intercostals, must, from the direction of the fibres 

 in relation to the direction of the portion of rib to which they are attached, 

 act, like the posterior portion of the external intercostals, in elevating the 

 ribs. (Here diagrams were employed to illustrate these points.) 



It is hence obvious that the combined influence of these portions of the 

 two muscles will be, where the fixed point is taken from the upper of the 

 two ribs to which they are attached, to swing the rib in an opposite 

 direction upon an imaginary axis drawn through the spinal and sternal 

 attachments, taking a true rib as a type. The entire length of the external 

 intercostals will also act with the levatores costarum, and produce the effect, 

 in forced inspiration, of raising the anterior end of the rib, and thus thrust 

 forward the sternum. 



In the contraction of the decussating portions of each set of muscles 

 simultaneously with the others, we get the diagonal of the action of both 

 as the result of their joint action, the upper of the two ribs to which each 

 pair of muscles is attached being relatively fixed. 



