MECHANICS OF RESPIRATORY MOVEMENTS 1167 

 by the previous inspiration, contract by virtue of the elastic tissue 

 they contain, and the thorax itself sinks by its own weight, and bv 

 the elastic reaction of the stretched costal cartilages. 



It must be remembered, however, that in a position of rest the 

 elasticity of the thorax is opposed to the elasticity of the lun*s. 

 Elasticity of the chest wall would therefore tend to produce inspiration! 

 This factor would tend to make inspiration easier at its onset, .but 

 would also present an impediment to the carrying out of expiration, 

 so that towards the end of this act there is need for the active co-opera- 

 tion of muscular contractions. It seems possible that more or less 

 muscular activity of the expiratory muscles is alternated with that of 

 the inspiratory muscles. In fact Sherrington's results on the co- 

 ordination of muscular movements would tend to make us assume 

 inhibition of the tone, e.g. of the abdominal muscles, during inspira- 

 tion, and active augmentation of their tone during expiration. Where 



FIG. 488. 



FIG. 489. 



the tone of the muscles is entirely lost, e.g. in the condition of viscero- 

 ptosis, it has been observed that the diaphragm is thrown out of action, 

 breathing being chiefly carried out by an elevation of the upper part 

 of the thorax. Probably under normal circumstances the internal 

 intercostal muscles also contract with each expiration. 



Although the action of the intercostal muscles has been a subject of debate, 

 physiological experiments serve on the whole to confirm the view first put 

 forward by Hamberger and based on a consideration of the direction of the 

 fibres. The external intercostals pass from one rib to the next below down- 

 wards and forwards. Hence if a pair of ribs be isolated from the rest of the 

 chest wall, leaving the vertebral and costal attachments intact, contraction of 

 these muscles will cause a rise of both ribs. This result will be evident from 

 a consideration of Fig. 488, where ab is a fibre of the external intercostal muscles, 

 passing from the rib vs to be attached to the rib v's' at b. When ab contracts, 

 the tension it exerts on its two attachments can be resolved into two components 

 ac acting downwards and bd acting upwards, bd, however, acts at the end of 

 the long lever bv', whereas ac acts at the end of a short lever av. Hence the 

 raising effect will overcome the depressing effect, and both ribs will rise. 



The fibres of the internal intercostals run in the opposite direction to the 

 external muscles, and from a consideration of Fig. 489 it is evident that their 

 effect will be to depress any pair of ribs, thus acting as expiratory muscles. 



Owing to the fact that the costal cartilages make an angle with the bony 



