MOVEMENTS OF THE THORAX. 165 



chief differences in the proportions appear below the third rib. The manubrium is 

 as large, relatively to the height, in one sex as the other, although the mesosternum 

 in women, especially its lower part, is less developed ; hence the ends of the car- 

 tilages of the lower sternal ribs are crowded together, and those of the seventh often 

 meet below the sternum, in front of the ensiform, thus practically lengthening the 

 body. The effect of this is that the relations of the viscera to the walls are not so 

 different in the sexes as one would expect.^ The floating ribs are small in women 

 and do not approach the pelvis so closely as in the male. The antero-posterior 

 diameter of the female chest is to the transverse as i to 2^ (subject to variation), 

 thus more resembling the proportions of the child. 



THE MOVEMENTS OF THE THORAX. 



The motions permitted by the following joints are to be considered separately, 

 although their interdependence is to be remembered. First, the joints of the verte- 

 bral ends of the ribs, the costo-central and the costo-transverse being taken together ; 

 second, those between the manubrium and gladiolus ; third, the costo-sternal and 

 interchondral joints ; fourth, as modifying these, flexion and extension of the spine ; 

 and fifth, the elasticity of the ribs and cartilages. 



Motions in the Costo-Vertebral Joints. — These vary greatly in different 

 parts of the column. The first rib moves as a hinge on a fixed axis running out- 

 ward, backward, and a little upward through the joint on the body of the verte- 

 bra and that on the transverse process. If this axis were strictly transverse, the 

 rising of the front of the rib would increase only the antero-posterior diameter of the 

 thorax, as the motion occurs in a plane at right angles to the axis. Since, however, 

 the axis is oblique, a plane at right angles to it extends forward and outward, and 

 motion in it thus increases also the transverse diameter of the chest. The shape of 

 the first rib is such that this transverse increase amounts to little or nothing, but this 

 principle comes into play with the longer ribs. The joint of the second rib is prac- 

 tically similar, except that the outer end of the axis at the tubercle is farther back, 

 so that the plane of motion slants more outward and the lateral expansion gained 

 by raising the second rib is more marked independently of the greater length of that 

 rib. With the third rib, usually, an important modification begins ; the outer end 

 of the axis is not fixed, for the tubercle slides on the transverse process. The 

 changes in the facets on the transverse processes have been described ; it appears 

 that, as we descend the spine, they are so placed and so shaped as to allow this 

 movement more and more freely. Thus, in the middle of the thoracic region the 

 outer end of the axis of rotation is so movable that the motion is to be decomposed 

 into two, — namely, one on the axis already described through the head and the 

 tubercle, and another on an antero-posterior axis passing through the head of the 

 rib and the joint between its costal cartila^ and the sternum. At the eighth rib of 

 the dissected spine a new motion appears, which becomes much more extensive 

 in the succeeding ones. The ligaments connecting the tubercle and neck to the 

 transverse process are less tense, and it is possible to move the tubercle a little 

 forward from its socket ; in the lower joints the rib can be moved upward, down- 

 ward, forward and backward, and circumducted. These motions are particu- 

 larly free at the last two thoracic vertebrae. Motion backward is checked by 

 contact with the transverse process ; forward, by the posterior and middle costo- 

 transverse ligaments ; upward motion of the last two ribs by the particularly strong 

 bands of fascial origin described with the ligaments ; downward motion by the in- 

 tercostal structures. An important deduction from this is that the last ribs can 

 be pulled downward and backward, so as to fix the posterior costal origin of the 

 diaphragm. 



Motions in the Intersternal Joints. — The joint between the manubrium 

 and the body of the sternum admits of motion on a transverse axis, which is free in 

 the young, but much restricted or abolished in the old. At rest, the two parts form 

 a slight angle open behind. This is effaced by the forward motion of the body on 



^ Henke : Arch, fiir Anat. u. Phys., Anat. Abtheil, 1883. 



