MOVEMENTS OF THE THORAX 247 



and they are connected at their margins by ligamentous tissue, which forms a complete capsule 

 for the articulation, and is Uned by a synovial membrane (fig. 282). The largest of these cavi- 

 ties is between the seventh and eighth; those between the eighth and ninth, and ninth and tenth, 

 are smaller, and are not free to play upon each other in the whole of their extent, being held 

 together by ligamentous tissue at theii* anterior margins. Sometimes this fibrous tissue com- 

 pletely obliterates the synovial cavity. 



The arteries are derived from the musculo-phrenic, and the nerves from the intercostals. 



Movements. — 'By means of the costal cartilages and interchondral joints, strength with 

 elasticity is given to the wall of the trunk at a part where the cartilages are the only firm struc- 

 tures in its composition; while a slight gliding movement is permitted between the costal carti- 

 lages themselves, which takes place round an axis corresponding to the long axis of the cartilages. 

 By this means, the outward projection of the lower part of the thoracic wall is increased by deep 

 inspiration. 



MOVEMENTS OF THE THORAX AS A WHOLE 



Before describing these movements as a whole, it must be premised that there are somlT, 

 few modifications in the movements of certain ribs resulting from their shape. Thus the firs' 

 rib (and to a less extent the second also), which is flat on its upper and lower surfaces, revolves 

 on a transverse axis drawn through the costo-vertebral and costo-transverse joints. During 

 inspiration and expiration, the anterior extremities of the first pair of costal arches play up and 

 down, the tubercles and the heads of the ribs acting in a hinge-like manner, the latter having 

 also a slight screwing motion. By this movement the anterior ends of the costal arches are 

 simply raised or depressed, and the sternum pushed a Uttle forward; it may be likened to the 

 movement of a pump-handle, as in fig. 283, a, b. 



The movements of the other ribs, particularly in the mid-region of the thorax, are more 

 complex, for, besides the elevation of the anterior extremities, the bodies and angles of the 



Fig. 283. — Diagram op Axis of Rib-movement. (After Kirkes.) 



ribs rise nearly as much as the extremities themselves. In this'movement the tubercles of the 

 ribs glide upwa.rd and backward in inspiration, and downward and forward in expiration; and 

 the movement may be likened to that of a bucket handle, as in fig. 283, A, B. 



During inspiration, the cavity of the thorax is increased in every direction. The antero- 

 posterior diameter is increased by the thrusting forward of the sternum, caused by the eleva- 

 tion of the costal cartilages and fore part of the ribs, whereby they are brought to nearly the 

 same level as the heads of the ribs. The transverse diameter is increased: (i) Behind, by the 

 elevation of the middle part of the ribs; for when at rest the mid-part of the rib is on a lower 

 level than either the costo-vertebral or chondro-sternal articulations. Owing to this obliquity 

 the transverse diameter is increased when the rib is raised, and the increase is proportionate 

 to the degree of obliquity, (ii) By the eversion of the lower border of the costal arch, which 

 turns outward as the arch is raised, (iii) The transverse diameter is increased in front by the 

 abduction of the anterior extremity of the rib at the same time as it is elevated and thrust 

 forward. 



The increase in the vertical diameter of the thorax is due to the elevation of the ribs, espe- 

 cially the upper ones, and the consequent widening of the intercostal spaces; but the chief 

 increase in this direction is due to the descent of the diaphragm. 



The greatest increase both in the antero-posterior and transverse diameters takes place 

 where the ribs are longest, most oblique, and most curved at their angles, and where the bulkiest 

 part of the lung is enclosed. This is on a level with the sixth, seventh, and eighth ribs. 



At the lower part of the thorax, where the ribs have no relation to the lungs, and do not 

 affect respiration directly by their movements, it is important that the costal arches should 

 be thrown well outward in order to counteract the compression of the abdominal viscera by the 

 contraction of the diaphragm. 



By widening and steadying the lower part of the thorax during inspiration, the attachments 

 of jthe muscular fibres of the diaphragm are widened, and their power increased. 



Muscles which take part in the movements of inspiration. — (a) Ordinary inspiration; The 

 acalenes, serratus posterior superior, the external and internal (?) intercostals, the diaphragm; 

 the quadratus lumborum and serratus posterior inferior fixing the lower ribs, possibly the poste- 

 rior fibres of the external oblique also helping to fix the lower ribs. (6) Extraordinary inspira- 

 tion: The superior extremities are raised and fixed. The cervical part of the vertebral colunan 

 and the head are extended, and in addition to the muscles of ordinary inspiration, the following 



