NEW VIEW OF THE MECHANISM OF RESPIRATION 293 



The muscles usually accredited with the production of calm or normal inspiration are — 



(a) The diaphragm. 



(b) The external and internal intercostals. 



(c) The minor pectoral muscles. 



(d) The levatores costarum. 



(e) The scaleni. 



To these I add the following correlated and co-ordinated muscles. The additions are jmnted in italics. 

 (/) The recti abdominis} 

 [g) The pyramidalis. 

 {h) The triangularis sterni. 

 (i) The external and internal oblique abdominis. 

 {]) The transversalis abdominis. 



When the muscles which fall imder a, b, c, d, e shorten or contract, those which fall under /, g, h, i, j elongate 

 or relax ; the two sets being complementary and working harmoniously to increase the capacity of the thorax in 

 all its diameters. 



In normal expiration the abdominal muscles shorten or contract ; the thoracic muscles, together with the 

 diaphragm, simultaneously elongating or relaxing. 



If the abdominal muscles did not elongate or relax when the thoracic muscles shortened or contracted, then 

 the thoracic ones would have forcibly to extend or drag out the abdominal muscles at each inspiratory effort, which 

 is more than they could possibly accomphsh. This follows, because the recti abdominis, pyramidahs, triangularis 

 sterni, and the external and internal oblique abdominis muscles directly depress the ribs and are the co-ordinates of 

 the external and internal intercostal muscles, the levatores costarum, and the scaleni, which directly elevate the ribs. 

 The inspiratory and expiratory muscles do not oppose or antagonise each other, as is generally supposed. On the 

 contrary, they work together to produce predetermined results, and they are so placed and so arranged that they 

 do what is required of them as prime movers and at first hand. 



Between the thoracic, abdominal, and diaphragmatic muscles there is a constant give and take which brings 

 about by a series of vital rhythmic movements an alternate increase and diminution of the thoracic and abdominal 

 cavities. 



According to accepted views, expiration in calm or normal respiration is not produced by muscular action at all. 

 It is said to be due chiefly to elastic recoil. The important function of expiration is left more or less to chance. The 

 elastic recoil mechanical theory of expiration is, on the face of it, erroneous. 



The accounts given of the respiratory movements of the nostrils, glottis, and chest are at once conflicting and 

 unsatisfactory. Thus in Quain's " Anatomy " ^ it is stated : " In normal and quiet expiration the diminution of the 

 capacity of the chest is mainly, if not wholly, due to the return of the walls of the chest to the condition of rest, 

 in consequence of their own elastic reaction and of the elasticity and weight of the viscera and other parts displaced by 

 inspiration." 



Similar remarks are made of the partial closing of the nostrils and the glottis in expiration. 



All this is highly perplexing. If the inspiratory and expiratory acts are co-ordinated and complementary, 

 which there is every reason to believe they are, they have necessarily the same value. It is irrational to assume 

 that inspiration is a -vital act produced by the shortening or contracting of living muscles while expiration is a purely 

 mechanical act due to elastic recoil. Moreover, the theory of elastic recoil, if appUed all round, is conflicting, inas- 

 much as it affirms that the several compartments of the heart are closed by vital acts and opened by mechanical 

 ones due to the elastic and other properties of the cardiac muscles, whereas the chest is opened by vital acts and 

 closed mainly or exclusively by elastic ones. Muscle is said to act in a certain way in one place and in another and 

 opposite way in another place. 



There are two formidable objections to the elastic recoil theory. 



First. There are muscles specially provided for producing the expiratory movements. These, as stated, are 

 the recti abdominis and pyramidalis, the triangularis sterni, the external and internal oblique abdominis, and the 

 transversalis abdominis. 



The recti abdominis and pyramidalis, the triangularis sterni. and the external and internal obUque abdominis, 

 when they shorten or contract, directly and at first hand pull down the sternum and ribs, and, aided by the elonga- 



1 The abdominal muscles are u.sually believed to have no function in calm ov normal inspiration, but only in forced or laboured expiration. 



" In forced expiration and in etlbrts of expulsion from the thorax and abdomen all the nuiscles which tend to depress the ribs, and those 

 which compress the abdominal cavity, concur in powerful action to empty the lungs, Hx the trunk, and expel the contents of the abdominal 

 viscera" (Quain's "Anatomy," vol. i. 13. 248). 



'' Seventh edition, vol. i. p. 249. 



