1044- 



THORAX. 



on the contrary, decrease in the degree of their 

 obliquity as they proceed in the same direc- 

 tion. Moreover, for the most part the ex- 

 ternal fibres increase in their obliquity in the 

 same intercostal space as you proceed from 

 the vertebrae towards the sternum, and the 

 internal intercostal fibres, on the contrary, 

 increase their obliquity from the sternum 

 towards the vertebrae, therefore they seldom 

 decussate at right angles to each other, or 

 form a perfect cross like the letter X. This is 

 their general relative position at death, but, 

 during life, in every stage of respiration, their 

 degree of obliquity varies. The obliquity of 

 the intercostal fibres should be viewed more 

 with reference to the spine than to the ribs, 

 because we shall show that their action is 

 relative to the spine, and not to the ribs 

 and that they may be perpendicular to two 

 ribs, while they are oblique to the spine, 

 because the ribs are themselves oblique. 

 We have never seen any of the external 

 intercostal fibres perpendicular to a rib, but 

 we may see that arrangement in the internal 

 layer of the lower intercostal spaces (fig. 

 673.). The omission of the relative posi- 

 tion of the spine with reference to the ob- 

 liquity of these muscles has led to many false 

 conclusions as to their action in respiration. 

 Let E E (fig. 675.) represent a spine or a rigid 



Fig. 675. 



body, and A c, B D two levers representing 

 ribs, allowing of free rotation at their centres of 

 motion A and B. These two bars are per- 

 pendicular, or at 90, with reference to the 

 body BE; let L K represent a connecting 

 tension or elastic fibre of any kind, this is 

 oblique with reference to the two bars A c 

 and BD, but move these bars down to the 

 position of p4, at an angle of 45 to the 

 body E E, and the fibre L K becomes perpen- 



dicular to the two bars ; now move the bars 

 up to p 3, also at an angle of 45 with E E, 

 and the fibre L K becomes more oblique than 

 at the position p 2. Therefore a tension 

 may change from the oblique to the perpen- 

 dicular relatively to the ribs ; but it can 

 never so change its relation to the spine. 

 Thus L" K" and i/ K' between the bars 

 at p 3 cross each other, in the same direc- 

 tion, but in different degrees of obliquity; 

 when the bars are at p 4., they decussate in 

 directions contrary with reference to the bars, 

 but not with reference to the body E E. The 

 position of the ribs is similar to the bars at 

 p 4, therefore the decussation of the inter- 

 costal muscles must be viewed with reference 

 to the spine. The intercostal fibres never 

 cross each other like the lines L'K' and 

 L"K" (fig. 675.), nor can they, by any change 

 of movement, ever assume thatposition ; t. e. 

 if they do not decussate in a direction con- 

 trary with relation to the spine, in no change 

 of position, throughout the range of a semi- 

 circle, can they ever become directly decus- 

 sating fibres but when they directly cross 

 each other as v D and V'B (fig. 676.) relative to 

 E E, in every other position to which the bars 



can be moved, they will be seen still to main- 

 tain the same decussation. 



Action of the intercostal muscles. There 

 is, probably, no subject in the whole range of 

 science which has excited more violent con- 

 tention and acrimonious dispute, than that 

 of the action of the intercostal muscles. The 

 illustrious and learned Haller could not enter 

 this field of inquiry without pausing to ob- 

 serve : " Let it be allowed me to deplore, 



