RESPIRATION 



393 



of bony arches attached 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 themselves and pursue 

 an oblique direction from above downward and forward. As a result the 

 anterior extremity lies at a lower level than the posterior. The costal 

 cartilages are directed upward and forward, with the exception of the upper 

 three, which are almost horizontal. 



The costo-vertebral and costo-chondral and the chondro-sternal articula- 

 tions are diarthrodial in character and endow the thoracic walls with a con- 

 siderable degree of mobility. The costo-vertebral joints are two in number, 

 the first being formed by the beveled head of the rib and the bodies of the 

 two adjoining vertebrae; the second, by the tubercle of the rib and the trans- 

 verse process. The costo-chondral and the chondro-sternal articulations, 

 as their names imply, are formed by the ribs, cartilages, and sternum. 



The muscles which complete the formation of the thoracic walls are 



Ext. Intercostal 



Int. Intercostal 



Inttrcartilaginei 



Levafores Costarum / 

 Extlntercostal 



Ni 



FIG. 186. SHOWING THE SITUATION, THE 

 POINTS OF ATTACHMENT, AND DIRECTION OF 

 THE INTERCOSTAL MUSCLES, i. The inter- 

 costales externi. 2. The intercostales in- 

 terni. 3. The intercartilaginei. 



MJntercostal 



FIG. 187. VIEW FROM BEHIND OF FOUR 

 DORSAL VERTEBRA AND THREE ATTACHED 



RlBS, SHOWING THE ATTACHMENT OF THE 



ELEVATOR MUSCLES OF THE RIBS AND THE 

 INTERCOSTALS. (After Allen Thomson.) 



as follows: the diaphram, the intercostales externi and interm, the levatores 

 costarum, the triangularis sterni, and the infra-costales. 



The diaphragm is the musculo-membranous sheet which closes the in- 

 ferior opening of the thorax and completely separates its cavity from ths 

 the abdomen It consists of two muscles which arise from the bodies of 

 the first three or four lumbar vertebrae and neighboring fascia fr 

 border of the six lower ribs, and from the ensiform cartilage. ^ 

 extensive origin the muscle-fibers pass centrally to be msertedmto a com 

 mon tendon As the direction of the fibers is from below .^^ and n- 

 ward, the diaphragm is somewhat dome-shaped. Its inferior border i 

 a short distance in contact with the sides of the -ax. 



The intercostales externi, eleven in number u n f eaC ^, Sld ';J r C r ?P y o 

 between the ribs to which they are attached from the tubercle 



186 and 187). Their fibers, which are arranged 



