RESPIRATION 



519 



to the vertebral column must be borne in mind. The head 

 of the rib is attached to the bodies of two adjacent vertebrae. 

 The tubercle of the rib is attached to the transverse process 

 of the hinder of these vertebrae. From this, the shaft of the 

 rib projects outwards, downwards, and backwards, to be 

 attached in front to the sternum by 

 the costal cartilage running for- 

 wards. If the rib is made to 

 rotate round its two points of 

 attachment, its lateral margin is 

 elevated and carried outwards, while 

 its sternal end is carried downwards 

 and forwards. 



The first pair of ribs does not 

 undergo this movement ; the motion 

 of the second pair of ribs is slight, 

 but the range of movement becomes 

 greater and greater as we pass down- 

 wards. This greater movement is 

 simply due to the greater length of 

 the muscles moving the ribs. The 

 muscles are the external intercostal 

 muscles, and they may be considered 

 as acting from the fixed first rib. 

 Now, if the fibres of the first inter- 

 costal muscle are one inch in 

 length, the second rib can be pulled 

 forwards, say, half an inch. The 

 first and second intercostals acting 

 on the third rib will together be two 

 inches in length, and, in contract- 

 ing, they can move the third rib 

 through, say, half of two inches 

 — i.e. one inch. The first, second, 

 and third intercostals, acting on 

 three inches in length, and can 

 rib half of three, or one and a half inches. The floating 

 ribs are fixed by the abdominal muscles, and limit the move- 

 ment of the ribs next above them. 



Fig. 210.— Rib and vertebral 

 column, .4., in an anterior 

 rib segment, and B., in a 

 posterior rib segment to 

 show the difiference in the 

 obliquity of articulation 

 and the resulting differ- 

 ence in the expansion of 

 the chest, which is greater 

 from side to side in the 

 more posterior part of the 

 chest. 



the fourth rib, are 

 therefore move this 



