39G EXTERNAL RESPIRATION 



sternal articulation. Each transverse process from above down- 

 wards is tilted a little more l^aekwards so that the angle of 

 articulation becomes more oblique as one passes down the 

 series. 



(iii.) Ligamentation. Each of the upper series of ribs is joined 

 directly to the sternum by a band of cartilage. The following are 

 the lengths of these attachments in a well-built man : second, 

 37 mm. ; third, 50 mm. ; fourth, 62 mm. ; fifth, 75 mm. The 

 angle of attachment increases as the length increases, e.g. the 

 second costal cartilage joins the sternum at right angles while the 

 third ascends to the sternum. 



(iv.) Musculature. The musculature of these ribs is the inter- 

 costal interchondral and external intercostal. 



(v.) Movements. Because of the nature of the articulation of 

 each rib to the vertebral column by tubercle and head, rotation 

 round a spino-sternal axis is limited. Very little bucket-handle 

 action can take place. As the articulations are practically trans- 

 verse, movement must occur at the manubrio-sternal articulation, 

 i.e. chiefly forwards. 



(c) The first rib provides the necessary fulcrum for the inter- 

 costal muscles. Along with the manubrium sterni, to which they 

 are firmly bound by their broad but short costal cartilages, the 

 first pair of ribs form the operculum or lid of the thorax. This 

 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 fibro-cartilage are bound together firmly by longitudinal 

 fibres developed from the strong and thick periosteum. The 

 limitation of movement thus imposed at the joint is counter- 

 balanced by the greater freedom of movement which is allowed at 

 the articulation of the heads of the first pair of ribs with the 

 thoracic vertebra. 



Great importance has been attached to the movements of this 

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

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

 abdominal w^all 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 

 l)reathing), 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 



