372 RESPIRATION. 



Nerve-supply. The lower intercostal nerves and the ilio- 

 inguinal nerve. 



Transversalis. It arises from Poupart's ligament, the crest of 

 the ilium, cartilages of the 6 lower ribs, and transverse processes 

 of the lumbar vertebrse, and is inserted into the linea alba or 

 pubes. 



Nerve-supply. The lower intercostal nerves. 



Rectus Abdominis. It arises from the os pubis, and is inserted 

 into the cartilages of the fifth, sixth, and seventh ribs. 



Nerve-supply. The lower intercostal nerves. 



Action. The internal intercostal muscles, except the anterior 

 portion (p. 368), depress the ribs, at the same time inverting their 

 lower borders, thus diminishing the size of the thoracic cavity. 



The triangularis sterni draws down the costal cartilages, thus 

 aiding in the expelling of air from the lungs. 



When the pelvis and the spine are fixed, the external and in- 

 ternal oblique, transversalis, and the rectus compress the thorax 

 at its lower part, and thus assist in the expiratory process. 



RESPIRATORY MOVEMENTS. 



The respiratory movements are of two kinds inspiratory and 

 expiratory. 



Inspiratory Movements. By virtue of the inspiratory 

 movements the air passes into the lungs. During their perform- 

 ance the thorax expands under the influence of the diaphragm 

 and the inspiratory muscles (p. 367). In inspiration all the diam- 

 eters of the chest are increased. The descent of the diaphragm 

 increases the vertical diameter (Plate 1). At the same time the 

 transverse and anteroposterior diameters are also increased. The 

 shape and direction of the ribs are such that when they are raised 

 their convexities are carried outward, and thus the transverse 

 diameter of the thorax is increased. But this movement also carries 

 the sternum forward, thereby increasing the anteroposterior diam- 

 eter. Under some circumstances, as when there is some obstruc- 

 tion to the entrance of air, additional muscles, called extraordinary 

 muscles of inspiration or muscles of forced inspiration (p. 369), are 

 brought into action. In this way most of the muscles about the 

 thorax may be called upon. It should be noted that inspiration 

 is an active process that is, one that requires for its performance 

 the action of muscles. 



Expiratory movements are for the most part passive in 

 their nature that is, are not due to muscular contraction. During 

 the descent of the diaphragm, referred to in describing the inspi- 

 ratory movements, the elastic abdominal organs and their attach- 

 ments and the abdominal walls are put upon the stretch. At the 

 end of the inspiratory act the diaphragm ceases to contract, and 



