198 THE MECHANISM OF 



dealing with the significance of the division of the lungs into lobes, 

 the direct influence of the diaphragm on the apices of the lungs 

 has been already pointed out. The upward movement of the 

 first pair of ribs and manubrium expand chiefly the anterior or 

 ventro -lateral part of the apex of the lung ; the movement has 

 but an indirect influence on the dorsal part of the apex, espe- 

 cially that part lying in front of the necks of the first and second 

 pair of ribs. It is the dorsal part of the apex of the lung that 

 is the common initial site of pulmonary tuberculosis. To secure 

 a free expansion of that area of the lung a full diaphragmatic con- 

 traction is much more effective than any movement of the upper 

 thorax. 



THE RESPIRATORY MOVEMENTS OF THE COSTAL CARTILAGES 



In man, and it is true of nearly all mammals, the costal carti- 

 lages progressively increase in length and in their declivity to the 

 sternum as one passes from the first downwards. The following are 

 the lengths of the cartilages in millimetres of a well-built man 

 from the first to the ninth ribs : first, 25 ; second, 37 ; third, 50 ; 

 fourth, 62; fifth, 75; sixth, 87; seventh, 112; eighth, 137; 

 ninth, 160. Although the eighth and ninth cartilages do not 

 directly reach the sternum, yet for functional purposes they really 

 do through their close union with the seventh costal cartilage, 

 and hence their .length is estimated as if they did reach the 

 sternum. The costal cartilage of the first rib descends to reach 

 the sternum ; that of the second normally lies horizontal and joins 

 the sternum at a right angle ; the third ascends to the sternum, 

 and the degree of ascent becomes greater with each succeeding 

 cartilage until, at the lower end of the sternum, the seventh pair 

 of costal cartilages form together the subcostal angle which varies 

 from 45 to 90, 65 being a common size in well-formed adults. 

 In observing the respiratory movements of any individual it is well 

 to regard the costal cartilages as a separate part of the respiratory 

 mechanism ; they have their own muscles ; during inspiration the 

 interchondral muscles (anterior part of the internal intercostals) 

 and the anterior digitations of the diaphragm raise them into a 

 more horizontal position, increasing the transverse diameter of the 

 thorax and the size of the subcostal angle. The cartilages are 

 depressed and the subcostal angle decreased by the upper part of 



