CHAP. VI.] THE THORAX. 143 



pieces of cartilage, which are of the same shape and breadth as the 

 ribs themselves. The ribs are articulated by their heads and tuber- 

 cles with the bodies, as well as with the transverse processes of the 

 vertebrae, and enjoy at these points a limited gliding motion in the 

 upward or downward direction. The direction of the true ribs is 

 forwards, sloping downwards ; the obliquity being greatest in the 

 lowest ribs, least in the first rib. The mobility which each rib 

 enjoys at its vertebral articulation, permits this direction to be 

 altered by muscular action ; and the ribs, under the influence of 

 their elevator muscles, pass from the sloping to the horizontal po- 

 sition. By this change the dimensions of the chest are enlarged 

 in the transverse as well as in the antero-posterior direction, for the 

 middle curved portions of the ribs are carried outwards, and there- 

 fore brought further apart from each other; and their sternal ex- 

 tremities are moved forwards, accompanied by the sternum, the dis- 

 tance of which from the dorsal vertebrae is thereby increased. The 

 forces, which depress the ribs, restore their planes of position to 

 their previous oblique direction, and the two diameters of the chest 

 to their former dimensions. It is scarcely needful to add, that the 

 elevation of the ribs accompanies inspiration, and their depression 

 expiration. 



The following happy comparison between the thorax and the 

 pelvis is from the pen of Mr. Mayo. 



"When we compare together the several regions of the trunk, 

 we observe that it is laid out in corresponding organs, or pairs of 

 organs, on either side of a centre, which is formed by the five lum- 

 bar vertebrae. Above the lumbar vertebrae are the dorsal ; above 

 these, the cervical: below the lumbar vertebrae are the sacral 

 bones ; below these, the coccygeal. To the dorsal vertebrae and to 

 the sacrum, bones are articulated, which have the double office of 

 forming a visceral cavity, and of throwing to a convenient distance 

 from the mesial plane the bones of the extremities. The ribs and 

 sternum, the clavicles and scapulae, form, with the dorsal vertebrae, 

 an organ strictly analogous to that formed by the ossa innominata 

 and the sacrum. But the chest for the function of respiration 

 requires to be continually altering its dimensions, and the upper 

 extremity is characterized by the extent and velocity of its move- 

 ments, rather than by strength : to suit both these objects, the 

 chest and shoulder are formed of many bones, that are moveable in 

 various senses; the ribs are capable both of rotating upon their 

 sternal and vertebral joints, and of being raised or depressed upon 

 their vertebral joints, carrying with them the sternum; .the clavicle 



