OF THE RIBS WITH THE VERTEBRA. 



237 



The Capsular Ligament is a thin, membranous sac attached to the circumference 

 of the articular surfaces, and enclosing a small synovial membrane. 



In the eleventh and tivelftli ribs this ligament is absent. 



Actions. The heads of the ribs are so closely connected to the bodies of the 

 vertebrae by the stellate and interarticular ligaments, and the necks and tubercles 

 of the ribs to the transverse processes, that only a slight sliding movement of 

 the articular surfaces on each other can take place in these articulations. The 

 result of this gliding movement with respect to the six upper ribs consists in an 

 elevation of the front and middle portion of the rib, the hinder part being pre- 

 vented from performing any upward movement by its close connection with the 

 spine. In this gliding movement the rib rotates on an axis corresponding with a 

 line drawn through the two articulations, Costo-central and Costo-transverse, which 

 the rib forms with the spine. With respect to the seventh, eighth, ninth, and tenth 

 ribs, each one, besides rotating in a similar manner to the upper six, also rotates 

 on an axis corresponding wiFh a line drawn from the head of the rib to the 

 sternum. By the first movement that of rotation of the rib on an axis corre- 

 sponding with a line drawn through the two articulations which this bone forms 

 with the spine an elevation of the anterior part of the rib takes place, and 

 a consequent enlargement of the 

 antero-posterior diameter of the 

 chest. None of the ribs lie in 

 a truly horizontal plane ; they are 

 all directed more or less obliquely, 

 so that their anterior extremities 

 lie on a lower level than their pos- 

 terior, and this obliquity increases 

 from the first to the seventh, and 

 then again decreases. If we ex- 

 amine any one rib say, that in 

 which there is the greatest obliq- 

 uity we shall see that it is ob- 

 vious that as its sternal extremity 

 is carried upward, it must also be 

 thrown forward ; so that the rib 

 may be regarded as a radius mov- 

 ing on the vertebral joint as a cen- 

 tre, and causing the sternal attach- 

 ment to describe an arc of a circle 

 in the vertical plane of the body. 

 Since all the ribs are oblique and 

 connected in front to the sternum 

 by the elastic costal cartilages, they 

 must have a tendency to thrust the 

 sternum forward, and so increase 

 the antero-posterior diameter of 

 the chest. By the second move- 

 ment that of the rotation of the 

 rib on an axis corresponding with 

 a line drawn from the head of the 

 rib to the sternum an elevation 



of the middle portion of the rib FlG . i62.-Diagrams showing the axis of rotation of the 



takpss nlapp anrl pnnpmipntl v an ribs in the movements of respiration. The one axis of rota- 



ce ' 8 >equentiy an tjon corresponds vvi th a line drawn through the two artieu- 



increase in the transverse diameter lations which the rib forms with the spine (a, 6) and the 



f, , , , ^ , .. , other vvith a line drawn from the head of the rib to the 



Ot tile Cnest. Jb Or the riDS not only sternum (A, B). (From Kirke's Handbook of Physiology.) 



slant downward and forward from 



their vertebral attachment, but they are also oblique in relation to their transverse 



plane that is to say, their middle is on a lower level than either their vertebral or 



