RESPIRATION 8/ 



of the thorax. In bringing about these movements the 

 following muscles are employed: 



1. The scaleni muscles, three in number on each side, which con- 

 nect at one end with the vertebras of the neck and at the other with the 

 first and second ribs. Their contraction slightly raises the upper 

 portion of the thorax. 



2. The elevators of the ribs, twelve in number on each side, which 

 are so distributed that each single muscle is attached, at one end, to the 

 back portion of a rib and, at the other, to a projection of the vertebra 

 a few inches above. The effect of their contraction is to elevate the 

 middle portion of the ribs and to turn them outward or spread them 

 apart. 



. 3. The intercostal muscles, which form two thin layers between the 

 ribs, known as the internal and the external intercostal muscles. The 

 external intercostals are attached between the outer lower margin of 

 the rib above and the outer upper margin of the rib below, and 

 extend obliquely downward and forward. The internal intercostals are 

 attached between the inner margins of adjacent ribs, and they extend 

 obliquely downward and backward from the front. The contraction of 

 the external intercostal muscles raises the ribs, and the contraction 

 of the internal intercostals tends to lower them. 



By slightly raising and spreading apart the ribs the 

 thoracic space is increased in two directions from front 



FIG. 42. Simple apparatus for illustrating effect of move- 

 ments of the ribs upon the thoracic space ; strips of card- 

 board held together by pins, the front part being raised or 

 lowered by threads moving through attachments at I and 2. 

 As the front is raised the space between the uprights is in- 

 creased. The front upright corresponds to the breastbone, the 

 back one to the spinal column, the connecting strips to the 

 ribs, and the threads to the intercostal muscles. 



to back and from side to side. Lowering and converg- 

 ing the ribs has, of course, the opposite effect (Fig. 42). 

 Except in forced expirations the ribs are lowered and con- 

 verged by their own weight and by the elastic reaction of 

 the surrounding parts, 



