THE THORAX 371 



Nervous Supply. The anterior thoracic nerves. 



Subclavius. It arises from the first rib and its cartilage, and is 

 inserted into the clavicle. 



Nervous Supply. A filament from the cord formed by the 

 union of the fifth and sixth nerves. * 



- Serratus Magnus. It arises from the 8 upper ribs and the 

 aponeurosis covering the intercostal muscles, and is inserted into 

 the scapula. 



Nervous Supply. The posterior thoracic nerve. 



Action. The trapezius and rhomboidei fix the scapula, and 

 the serratus magnus, contracting, raises the ribs. The arm being 

 fixed, the latissimus dorsi also raises the ribs. The ribs are like- 

 wise elevated by the action of the serratus posticus superior, while 

 the serratus posticus inferior draws downward and backward the 

 lower ribs, increasing thereby the capacity of the thorax. Some 

 authorities regard the serratus posticus superior as being brought 

 into action in ordinary respiration. When the ribs are drawn 

 downward they are held there by the serratus posticus inferior, 

 thus overcoming the upward lifting of the ribs by the diaphragm. 

 The iliocostalis and quadratus lumborum fix the last rib and oppose 

 the tendency of the diaphragm to raise it. The head being fixed, 

 the sternomastoid elevates the thorax. The group of muscles 

 consisting of the pectoralis major and minor and the subclavius 

 draw the ribs upward when the head is fixed. 



In this manner all the muscles mentioned, some to a greater 

 and some to a lesser degree, aid in the process of forced inspira- 

 tion. 



Muscles of Forced Expiration. Internal intercostals, triangularis 

 sterni, obliquus externus, obliquus internus, transversalis, rectus. 



Internal Intercostals (p. 368). 



Triangularis Sterni. This muscle is situated on the back of 

 the sternum and anterior portion of the ribs. It arises from the 

 sternum, ensiform cartilage, and costal cartilages of the 3 or 4 

 lower true ribs, and is usually inserted into the costal cartilages 

 of the second, third, fourth, and fifth ribs. 



Nerve-supply. The intercostal nerves. 



Obliquus Externus. This muscle is more familiarly known as 

 the external oblique. It arises from the 8 lower ribs, and is inserted 

 into the crest of the ilium and into tendinous fibers which form a 

 broad aponeurosis. 



Nerve-supply. The lower intercostal nerves. 



Obliquus Internus. This is also called the internal oblique. It 

 arises from Poupart's ligament, the crest of the ilium, and the 

 posterior lamella of the lumbar fascia, and is inserted into the os 

 pubis, linea alba, through an aponeurosis to the cartilages of the 

 seventh, eighth, and ninth ribs, and into the cartilages of the tenth, 

 eleventh, and twelfth ribs. 



