OF THE RIBS WITH THE VERTEBRA. 



333 



spending facets. By the first movement 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 

 takes place, and consequently an 

 increase in the transverse diameter 

 of the chest. This elevation of 

 the 3d. 4th, 5th, and 6th ribs is 

 due entirely to the shapes of the 

 ribs i. e. each rib being bent or 



twisted around three axes and not to this movement (see above). For 

 the 7th, 8th, 9th, and 10th ribs this elevation is due both to their shapes 

 and to this movement. The last two ribs move chiefly backward and for-' 

 ward, and with very little " elevation " of their middle portions (see Fig. 

 237). The mobility of the different ribs varies very much. The first rib is 

 more fixed than the others, on account of the weight of the upper extremity 

 and the strain of the ribs beneath ; but on the freshly dissected thorax it moves as 

 freely as the others. From the same causes the movement of the second rib is 

 also not very extensive. In the other ribs this mobility increases successively 

 doAvn to the last two, which are very movable. The ribs are generally more 

 movable in the female than in the male. 



VII. Articulation of the Cartilages of the Ribs with the Sternum, 



etc. (Fip. 238). 



The articulations of the cartilages of the true ribs with the sternum are artb.ro- 

 dial joints, with the exception of the first, in which the cartilage is almost always 

 directly united w r ith the sternum, and which must therefore be regarded as a 

 synarthrodial articulation. The ligaments connecting them are 



FIG. 237. Diagrams showing the axis of rotation of the 

 ribs in the movements of respiration. The one axis of rota- 

 tion corresponds with a line drawn through the two articu- 

 lations which the rib forms with the spine (a, b), and the 

 other with a line drawn from the head of the rib to the 

 sternum (A, B). (From Kirke's Handbook of Physioloyy.) 



