RESPIRATION. 



34i 



their associated structures are lodged, is conic in shape, though some- 

 what compressed from before backward. Its apex is directed up- 

 ward, its base downward. The thoracic framework is formed 

 posteriorly by the thoracic vertebrae and the posterior extremities 

 of the ribs, laterally by the ribs, and anteriorly by the costal car- 

 tilages and the sternum. The superior opening, through which 

 pass the trachea, esophagus, and blood-vessels, is oval in outline 

 and measures from side to side about 12.5 cm., and from before 

 backward about 6.25 cm. The inferior opening is of large size, but 

 irregular in its boundaries from 

 the upward inclination of the ribs 

 and the downward projection of 

 the sternum. 



The ribs which form a large 

 part of the thoracic walls consti- 

 tute a series of bony arches at- 

 tached posteriorly to the vertebrae 

 and anteriorly to the sternum 

 through the intermediation of 

 their cartilages. The last two 

 form an exception. The ribs are 

 somewhat twisted upon them- 

 selves and pursue an oblique 

 direction from above downward 

 and forward. ^A.s a result the 

 anterior extremity lies at a lower 

 level than the posterior^- The 

 costal cartilages are directed up- 

 ward and forward, with the ex- 

 ception of the upper three, which 

 are almost horizontal^ The gen- 

 eral arrangement and appearance 

 of the thorax are shown in Fig. 

 164. 



The costo-vertebral and costo- 

 sternal articulations are diar- 



throdial in character and permit of an alternate elevation and depres- 

 sion of the ribs both anteriorly and posteriorly. The resultant of 

 all the movements permitted by these joints is an elevation of the 

 ribs and an advance of the sternum, and a consequent increase in the 

 transverse and antero-posterior diameters. Between and over the 

 ribs lie muscles, which are in turn covered by fascia and skin. 



The diaphragm, which closes the inferior opening of the thorax 

 and completely separates its cavity from that of the abdomen, is a 

 musculo-membranous sheet. It consists of two muscles which arise 



\ 



FIG. 164. THORAX, ANTERIOR VIEW. 

 i. Manubrium sterni. 2. Gladio- 

 lus. 3. Ensifortn cartilage of 

 xiphoid appendix. 4. Circumference 

 of apex of thorax. 5. Circum- 

 ference of base. 6. First rib. 7. 

 Second rib. 8, 8. Third, fourth, 

 fifth, sixth, and seventh ribs. 9. 

 Eighth, ninth, and tenth ribs. 10. 

 Eleventh and twelfth ribs, n, n. 

 Costal cartilages. 



