512 



OF RESPIRATION. 



so soon as it has become relaxed. There are many accessory muscles, however, 

 which take a share in violent respiratory movements, both inspiratory and ex- 

 piratory. Thus all the muscles which elevate the scapula may act through it 

 upon the ribs, and the scaleni act directly upon the first rib ; whilst all those 

 which erect the spine, fix more perfectly the origins of these and other muscles 

 which are to act upon the thorax. So, again, the expiratory movement is aided 

 by the longissimus dorsi, sacro-lumbalis, and other muscles which tend to de- 

 press the ribs. In difficult respiration, almost every muscle in the body is made 

 in some way subservient to the distension of the chest; thus, a patient suffering 

 under urgent dyspnoea instinctively lays hold of some fixed object, so as to pre- 

 vent his upper extremities from moving; and thus his scapula becomes a fixed 

 point, from which the pectorales (major and minor) and serratus magnus can 

 aid in elevating the ribs. 



545. The relative amount of muscular force which is required for these two 

 movements respectively, is affected in a very remarkable manner by the elasti- 

 city of the walls of the thoracic cavity; for this (like the elasticity of the lungs), 

 supplies a force which greatly aids the expiratory movement, whilst it offers a 

 corresponding opposition to the mspiratory. Here, also, the degree of force 

 exerted increases very rapidly with the degree of distension. Thus in a body 

 experimented on by Dr. Hutchinson (Op. cit. p. 1056), the following were the 

 relations between the amount of air forced in, the resisting elasticity, as shown 

 by the height of mercury supported, the actual pressure upon each square inch 

 of surface which this indicated, and the total pressure over the surface of the 

 chest, reckoning its area at 206 square inches : 



Air forced in 



To this 451.9 Ibs. must be added at least 128 Ibs. for the elastic force of the 

 lungs themselves at that degree of distension, making altogether 580 Ibs. ; and 

 as the subject of this observation could expire during life considerably more air 

 than the highest amount forced into his chest after death, there can be little 

 doubt (judging from the rapid ratio in which the elastic force increases when 

 the distension is approaching its limits) that the muscular power required to 

 overcome this, towards the close of a very deep inspiration, could not have been 

 less than 1000 Ibs. The co-operation of the ilastic resistance with the expira- 

 tory movement, and its antagonism to the inspiratory, is doubtless the principal 

 cause why the power of the expiratory muscles, as tested by the height of the 

 column of mercury supported by the air, should always be greater than that of 

 the inspiratory muscles (see Dr. Hutchinson, Op. cit. p. 1061) ; and why the 

 expiratory power should be very much greater when the chest has been well 

 filled with air, than when it is comparatively empty. The following is given by 

 Dr. Hutchinson as the range through which these powers may vary within the 

 limits of health : 



