308 EXTERNAL RESPIRATION 



lid is articulated anteriorly with the thoracic wall, at the manubrio- 

 sternal joint, forming a synchondrosis. That is, the opposing 

 surfaces of bone covered with a layer of hyaline cartilage and 

 united by fibre-cartilage are bound together firmly by longitudinal 

 fibres developed from the strong and thick periosteum. The 

 cartilages of the first ribs are implanted upon the side of the 

 mamibrium forming a synchondrosis with the sterno-manubrial 

 joint and there is no synovial cavity. 



Great importance has been attached to the movements of this 

 joint. Its amplitude varies, of course, with the type of respiration, 

 being greatest with those who make least use of the muscles of the 

 abdominal wall and vice versa. In other words, if the sternum 

 moves freely then the excursions of the sterno-manubrial joint 

 will be small. On the other hand, in cases where the lower part 

 of the sternum moves but little during inspiration (thoracic 

 breathing), there will be a correspondingly large rotation of the 

 upper end of the sternum on the end of the manubrium. Some 

 physicians declare that in phthisical subjects this joint does not 

 move freely. Whether phthisis causes an anchylosis or whether 

 want of free movement leading to incomplete expansion of the 

 apices of the lungs is a factor favouring the development of the 

 disease, is as yet an unsolved problem. On the whole the evidence 

 tends to show that ossification of the costal cartilages in question 

 is a consequence rather than a cause of a limited expansion of the 

 apices of the lungs. 



Posteriorly the lid is articulated to the vertebral column by a 

 joint which is set more transversely and is wider in the extent of 

 its attachment than any other of the costal arcs. 



IV. Mechanics of Thorax. The ribs are a series of bent levers. 



(1) The fulcra or hinges on which the levers work have been 

 inentioned when dealing with the ribs of the various thoracic 



' segments. 



(2) The power differs according to whether inspiration or 

 expiration is being performed (p. 320). 



(a) Inspiration. 



(i) The lid or operculum is raised by the action of a flat triangular 

 muscle (scaleni). The scalenus anticus is inserted in the upper 

 surface of the first rib just dorsal to the cartilage and passes almost 

 vertically to the transverse processes of the third, fourth, fifth and 

 sixth cervical vertebrae. The scalenus medius lies dorsally to the 

 anticus and passes to the transverse processes of the lower six 

 cervical vertebrae. 



