THE THORAX 1045 



and it may be the 5th, and even the loth. The lower border of 

 each cartilage projects downwards, and comes into contact wath 

 the upper border of the cartilage below. Each joint is surrounded 

 by fibres disposed in the form of a capsule, and it is provided with 

 a sjTiovial membrane. 



Arterial Supply. — The musculo-phrenic branch of the internal 

 mammary arterj'. 



Nerve-supply. — The adjacent intercostal nerves. 



3. Sternal Articulations. — The joint between the manubrium and 

 body of the sternum belongs to the class amphiarthrosis, and sub- 

 division symphysis. The opposed surfaces are covered by hyaline 

 cartUage, and a plate or disc of fibro-cartilage is interposed between 

 them, which is connected at either side with the interarticular hga- 

 ment of the second chondro-stemal joint. This disc may contain 

 a small cavity. In front of and behind the joint there are Hga- 

 mentous fibres which are disposed longitudinally. 



The entire sternum is strengthened by its dense periosteum, by 

 the radiating fibres of the chondro-stemal hgaments, and by the 

 tendinous fibres of origin of the pectoralis major muscles. 



Movements ol the Ribs. — The movement allowed at the eosto-eentral and 

 costo-transverse joints is of a gliding nature, and takes place in an upward 

 and do%\Taward direction. During this movement the rib rotates round the 

 eosto-vertebral axis, which corresponds to a line passing obliquely through 

 the costo-central joint, the neck of the rib, and the costo-transverse joint. 

 0\\'ing to the curve and downward inclination of the rib, the result of this 

 rotation is that the anterior and lateral parts of the rib are elevated. Simul- 

 taneously, on account of the obliquity of the axis of rotation, the anterior 

 part of the rib is carried forwards, and along with it the sternum, thus in- 

 creasing the antero-posterior diameter of the thorax. The lateral part of the 

 rib, and to a certain extent the anterior part also, are carried outwards, thus 

 giving rise to eversion of the lower border of the rib, and at the same time 

 the angle between the rib and its costal cartilage is opened out. In this 

 manner an increase in the transverse diameter of the thorax is produced. 

 During the elevation of the anterior and lateral parts of the rib and the ever- 

 sion of its lower border the movement takes place round the costo-sternal 

 axis, which corresponds to a line drawn from the costo-central joint of one 

 side to the corresponding chondro-stemal joint. It is usual to liken this 

 movement to the movement of the handle of a bucket. 



In the case of the first rib elevation and depression are the chief move- 

 ments allowed, the amount of eversion being trivial, inasmuch as the axis 

 of rotation is almost transverse. In the case of the second, third, fourth, 

 fifth, and sixth ribs elevation and depression, along with eversion, are allowed, 

 the axis of rotation in each case becoming successively more oblique. The 

 seventh, eighth, ninth, and tenth ribs, in which the costo-transverse articular 

 surfaces are almost flat, besides rotating round the costo-vertebral axis, also 

 rotate round the costo-sternal axis. In the case of these ribs elevation is 

 accompanied by a backward movement, and depression by a forward move- 

 ment. These backward and forward movements take place more freely in 

 the case of the eleventh and t%velfth ribs, which have no costo-transverse 

 joints. At the chondro-stemal joints the movement is limited, and consists 

 in elevation and depression, together with forward and backward movement. 



At the interchondral joints slight gliding movement is allowed. 



Muscles concerned in Respiration. — In ordinary quiet inspiration the 

 muscles concerned are as follows : (i) the diaphragm ; (2) the external and 

 internal intercostal muscles, assuming Haller's view to be correct ; (3) the 



