368 RESPIRATION. 



first rib. The scalenus medius arises from the posterior tuber- 

 cles of the transverse processes of the cervical vertebrae from the 

 second to the seventh, both inclusive, and is also inserted into the 

 first rib. The scalenus posticus arises from the posterior tuber- 

 cles of the transverse processes of the fifth, sixth, and seventh 

 cervical vertebrae, and is inserted into the second rib. 



Nerve-supply. Branches of the anterior divisions of the fifth, 

 sixth, seventh, and eighth cervical nerves. The scalenus medius 

 receives an additional supply from the deep external branches of 

 the cervical plexus. 



Intercostal Muscles. These fill up the intercostal spaces, and 

 consist of muscular and tendinous fibers, the combination of the 

 two kinds of tissue giving both contractility and strength. There 

 are two sets of these muscles, external and internal. 



External Intercostal Muscles. These fill the intercostal spaces 

 from the tubercles of the ribs to the costal cartilages, from which 

 point to the sternum there is no muscular tissue. They arise from 

 the lower borders of the ribs, and are inserted into the upper 

 borders of the ribs below them. The direction of their fibers is 

 obliquely downward and forward. 



Nerve-supply. The intercostal nerves. 



Internal Intercostals. Those Avhich are attached to the true 

 ribs extend 'from the sternum to the angles of the ribs, where the 

 muscular tissue ceases to exist and a membranous structure takes 

 its place as far as the vertebrae. Those which are attached to the 

 false ribs extend from their cartilages backward in a manner 

 similar to that just described. The fibers of this group arise from 

 the ridge on the inner surface of the ribs and from the costal 

 cartilages, and are inserted into the upper borders of the ribs 

 below. The direction is obliquely downward and forward, the 

 external and internal intercostals, therefore, cross each other. 



Nerve-supply. The intercostal nerves. 



Levatores Costarum. These muscles arise from the extremities 

 of the transverse processes of the vertebrae from the seventh 

 cervical to the eleventh dorsal, and are inserted into the upper 

 borders of the ribs between the tubercles and the angle. 



Nerve-supply. The intercostal nerves. 



Action. The first rib on each side is raised and held in a fixed 

 position by the scaleni muscles ; the external intercostals now 

 contracting, all the ribs are raised. This elevation of the ribs is 

 assisted by the contraction of that portion of the internal inter- 

 costals which is situated in the front of the thorax, and by that of 

 the levatores costarum. These are, therefore, all muscles of ordi- 

 nary inspiration. 



All authorities are not agreed as to the action of the inter- 

 costals. Haller regarded both external and internal intercostals 

 as muscles of inspiration ; Keen considers the external intercostals 



