210 STUDIES IN ADVANCED PHYSIOLOGY. 



more largely with the diaphragm, while that in women is 

 more largely with the ribs. 



The muscles that move the ribs up and down are called 

 the intercostal muscles. Of these muscles there are two 

 sets, the external intercostals and the internal intercostals. 

 The external intercostals are so arranged that the insertion 

 point on the lower rib is always forward from the insertion 

 on the upper rib, while in the internal intercostals the re- 

 verse is true. To show how the contraction of the ex- 

 ternal intercostals raise the ribs and breast bone, while the 



Fig. 91. DIAGRAM TO ILLUSTRATE THE MANNER OF ENLARGEMENT OF CHEST. 



internal intercostals pull it down, a little piece of apparatus 

 may be constructed such as the one indicated in the accom- 

 panying diagram. If, now, a piece of rubber be stretched 

 in such a way as to correspond with the pull of the ex- 

 ternal intercostals it will raise the bars. When arranged 

 like the internal intercostals it will pull it forcibly down- 

 wards. That such would result is of course easily seen by 

 studying Figure 92. The line a b' , corresponding to the ex- 

 ternal intercostals is shortened, which, of -course, is analo- 

 gous to the contraction of the muscle itself. Shortening can 

 take place only when both beams are pulled upward, while 

 the exactly opposite is the result with the internal inter- 

 costals. 



2. The Rate of These Movements. The rate at which 

 inspirations follow each other varies considerably under 

 different circumstances, but on an average in an individual 



