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This question being at rest, although in pursuit of science one should 

 inquire how things are effected, and not, wherefore they come to pass, one 

 feels naturally desirous to know what purpose is answered by the different 

 direction of the fibres of the two sets of intercostal muscles; and with 

 what view Nature has departed from her wonted simplicity, in giving to 

 their fibres opposite directions. In answer to this, one may observe, that 

 the action of powers applied obliquely to a lever, being decomposed in 

 consequence of that obliquity, a part of the action of the external inter- 

 costals would tend to draw the ribs towards the vertebral column, which 

 could not happen, without forcing back the sternum, if the internal inter- 

 costals did not tend to bring forward the ribs, at the same time that they 

 elevate them; so that these two muscular planes, united in their action 

 of raising the ribs, antagonise and reciprocally neutralize each other, in 

 the effort by which they tend to draw them in different directions. 



To this advantage of mutually correcting the effects that would result 

 from their respective obliquity, may be added the benefit arising from a 

 texture capable of a greater resistance ; it is clearly obvious, that a tissue 

 whose threads cross each other, is firmer than one in which ail the 

 threads merely in juxta position, or united by means of another substance, 

 should all lie in the same direction. Hence, Nature has adopted this 

 arrangement, in the formation of the muscular planes constituting the 

 anterior and lateral parietes of the abdomen, without which the abdomi- 

 nal viscera would frequently have formed herinary tumours, by sepa- 

 rating the fibres and getting engaged between them. In this respect, one 

 may compare the tissue of the abdominal parietes, in which the fibres of 

 the external and internal oblique muscles, which cross each other, are 

 themselves crossed by the fibres of the transversales, to the tissue of those 

 stuffs whose threads cross each other, or rather to wicker work, to which 

 basket-makers give so much strength, by interweaving the osier in a va- 

 riety of directions. 



LXXIII. When from any cause, respiration becomes difficult, and the 

 diaphragm is prevented from descending towards the abdomen, or the 

 motion of inspiration is impeded, in any way, the intercostals are not alone 

 employed in dilating the chest, but are assisted by several other auxiliary 

 muscles; the scaleni, thesubclavii, the pectorales, the serrati inagni, and 

 the latissimi dorsi, by contracting, elevate the ribs, and increase, in more 

 directions than one, the diameter of the chest. The fixed point of these 

 muscles, then, becomes their moveable point, the cervical column, the 

 clavicle, the scapula, and the humerus, being kept fixed by other powers, 

 which it is unnecessary to enumerate. Whoever witnesses a fit of con- 

 vulsive asthma, or of a suffocating cough, will readily understand the im- 

 portance and action of these auxiliary muscles. 



Inspiration is truly a state of action, an effort of contractile organs, 

 which must cease when these are relaxed. The expiration which follows 

 is passive, and assisted by very few muscles, and depends chiefly on the 

 re-action of the elastic parts entering into the structure of the parietes 

 of the chest. We have seen, that the cartilages of the ribs are pretty 

 considerably twisted, so as to carry outward and downward their upper 

 edge : when the cause which occasions this twisting ceases to act, these 

 parts return to their natural condition, and bring back the sternum to- 

 wards the vertebral column, towards which the ribs descend, from their 

 weight. The diaphragm is forced towards the chest, by the abdominal 

 viscera, which are compressed by the broad muscles of the abdomen. 



