290 PULMONARY MUCOUS TISSUE. 



column, and representing the cord of the bow formed by the 

 rib : the convexity of the rib is thus turned outwards, causing 

 a transverse dilatation of the thorax. 



The muscles which communicate these motions to the ribs 

 are well known ; they are those of the walls of the thorax, 

 and their action is demonstrated by simply studying the 

 direction of their fibres. They do not always act, however. 

 When the breathing is calm, as it usually is, contraction of 

 the intercostals, the scaleni, and, perhaps, a portion of the 

 serratus magnus and of the serratus posticus superior, etc., 

 will suffice ; but, if the inspiration becomes forcible, and, as 

 it were, constrained, we find (in cases of dyspno3a, for in- 

 stance) that the sterno-cleido-mastoideus, the pectoral, the 

 latissimus dorsi, and those muscles in general which, acting 

 from a fixed position (especially when the arms are elevated 

 and fixed) serve to raise the ribs and the sternum ; all these 

 come in play as re-enforcements. We shall also see that the 

 diaphragm even may assist in the elevation of the ribs. 



The working of these muscles may be easily observed in 

 a single anatomical inspection. This is not the case, how- 

 ever, with the intercostal muscles, which have always been a 

 subject of keen discussion among physiologists. We know 

 that these muscles are divided into internal intercostal and 

 external intercostal muscles, the fibres of each arranged cross- 

 wise. Every possible suggestion has been made as to the 

 mode of action of these muscles, which have been thought 

 to possess the power of inspiration and expiration, or one or 

 the other only. 1 To our mind, the intercostal muscles per- 



1 Beau and Maissiat (Archives Ge'ndrales de Medecine, 1812, 

 1843) have drawn up a curious list of the theories entertained as 

 to the functions of the intercostal muscles. The ten theories have 

 each been defended by numerous physiologists from Hamberger and 

 Haller to Beau, Maissiat, and Sibson Since that time (184:5) 

 other physiologists have taken part in this still undecided and 

 apparently fruitless discussion. These theories may be summed 

 up, by dividing them, as is done by Sappey, into six classes: 

 1. The external and internal intercostal muscles are both inspirator^ : 

 Borelli, Senac, Boerhaave, Winslow, Haller, Cuvier, Duchenne (de 

 Boulogne), Marcellin Duval. The latter bases his opinion on ex- 

 periments made on executed criminals a short time after death, 

 when the muscles were still excitable. Duchenne (de Boulogne) 

 rests chiefly on clinical observations made in cases of paralysis, in 

 which respiration was kept up, in spite of the respiratory muscles 

 being paralyzed, showing that active inspiration must have taken 

 place by means of the intercostal muscles. We remark, in all the 



