368 



MUSCLES AND FASCIAE. 



costals in raising the ribs. The Triangularis Sterni draws down the costal car- 

 tilages ; it is therefore an expiratory muscle. 



Muscles of inspiration and expiration. The muscles which assist the action of 

 the Diaphragm in inspiration are the Intercostals and the Levatores Costarum 

 as above stated, the Scaleni, the Serratus Posticus Superior, and to a slight 

 extent the Subclavius. When the need for more forcible action exists, the 

 shoulders and the base ot the scapula are fixed, and then the powerful muscles 

 of forced inspiration come into play ; the chief of these are the Serratus Mag- 

 nus, Latissimus Dorsi, and the Pectorales, particularly the Pectoralis Minor. 



The ordinary action of expiration is merely passive, the resilience of the ribs 

 and the elasticity of the lungs producing a tendency to a vacuum. This causes 

 the ascent of the abdominal viscera covered by the Diaphragm. Forced expi- 

 ratory actions are performed mainly by the flat muscles (Obliqui and Trans- 

 versalis) of the abdomen, assisted also by the Rectus. Other muscles of forced 

 expiration are the Internal Intercostals and Triangularis Sterni (as above men- 

 tioned), the Serratus Posticus Inferior, the Quadratus Lumborum and the Sacro- 

 lumbalis. 



DlAEHRAGMATIC REGION. 



Diaphragm. 



The Diaphragm (Aia'tpoyiua, a partition wall) (Fig. 234) is a thin musculo-fibrous 

 septum, placed obliquely at the junction of the upper with the middle third of 



Fig. 234. The Diaphragm. Under Surface. 



the trunk, and separating the thorax from the abdomen, forming the floor of the 

 former cavity and the roof of the latter. It is elliptical, its longest diameter 



