ELEMENTARY PHYSIOLOGY 



The internal intercostal muscles depress the ribs, thus making 

 the chest narrower and assisting expiration. 



The student may construct for himself the simple apparatus illustrated in 

 fig 5 - J 37> I 3%> and 139, which will at once make clear the action of the 

 intercostal muscles. A B is an upright bar of wood which represents the 

 spine. With this are jointed two parallel bars, C D and E F, which represent 

 two of the ribs. Another bar, D F, represents the sternum. The joints at 

 C, D, E, and F are all movable. An elastic band is now stretched obliquely 

 between these parallel bars thus representing the intercostal muscles. If the 

 band passes downwards and backwards (i.e. towards the spine) as in fig. 138, 



Fig. 136. Intercostal Muscles. 



iv', v',and vi', 4th, 5th, and 6th dorsal vertebrae ; v, vi, and vn, sth, 6th, and 7th ribs ; 5, 6, and 

 7, sth, 6th, and 7th costal cartilages ; i, sternum ; 2, external intercostal muscles ; 3, internal 

 intercostal muscles, the external layer having been removed. 



it represents the internal intercostals. As it contracts it tends to bring its 

 two ends as near together as possible, and in doing this it pulls down the bars 

 representing the ribs. By stretching the band downwards and forwards, as 

 in fig. 139, we illustrate the action of the external intercostals, which by their 

 contraction raise the ribs. 



We are now better able to understand the respiratory move- 

 ments, which may be briefly described as follows : 



During inspiration the diaphragm descends, making the 

 chest deeper ; and the external intercostals contract, thus raising 

 the ribs and making the chest wider. The capacity of the chest 

 being thus enlarged, the lungs are inflated as before described. 



