ACTION OF THE INDIVIDUAL RESPIRATORY MUSCLES. 



213 



II. During forced expiration the following muscles are employed : 



1. The abdominal muscles (internal or anterior abdominal nerves, branches of 

 the intercostal nerves from the 8th to the i2th). 



2. Internal intercostal muscles (the parts lying between the ribs), and the 

 infracostal muscles (intercostal nerves). 



3 . The triangular muscle of the sternum (intercostal nerves) . 



4. (?) Posterior inferior serratus (external branches of the dorsal nerves). 



5. (?) Quadratus lumborum (muscular branches of the lumbar plexus). 



ACTION OF THE INDIVIDUAL RESPIRATORY MUSCLES. 



A. Inspiration. i. The diaphragm arises by six processes from the six lower 

 costal cartilages and contiguous osseous parts of the ribs (costal portion) , by three 

 processes from the upper four lumbar vertebrae (lumbar portion) , and from the ensi- 

 form process (sternal portion). It presents a double dome, with the convexity 

 toward the thoracic cavity, and contains the liver in its larger, right-sided con- 

 cavity, and the stomach and the spleen in its smaller, left-sided cavity. In a state 

 of rest the intra-abdominal pressure and the elasticity of the abdominal wall press 

 these organs against the under surface of the diaphragm, in such a manner 

 that it bulges into the thoracic cavity. This position is aided by the elastic 

 traction of the lungs. The central part of the diaphragm (central tendon) is, to 

 a great extent, fused on its upper surface with the pericardial sac. This part, 

 which supports the 

 heart and is pierced 

 by the inferior vena 

 cava (foramen quad- 

 rilaterum) , projects 

 downward into the 

 abdominal cavity in 

 a state of rest ; and in 

 casts made of the 



FIG 80. Frontal Section of the Thorax at the Extremity of the Twelfth Rib on 

 Each Side (12. C), to Demonstrate the Form of the Diaphragm during Ex- 

 piration (Z e-Z e) and during Inspiration (Z i-Z i) : T e-T e, thoracic wall in 

 a state of expiration; i i, during inspiration; C t, central tendon. The ar- 

 rows indicate the direction of the movements during inspiration. 



diaphragm it can be 

 recognized as the 

 lowest part of the 

 middle portion (Fig. 



80). 



During c o n- 

 traction of the dia- 

 phragm the two 

 dome-like projec- 

 tions are flattened, 

 and the thoracic 

 cavity is enlarged 

 downward. At the 

 same time the dis- 

 tal, arched, muscular parts become flatter, and are drawn away from 

 the chest-wall, to which they are closely applied during expiration. The 

 middle part of the central tendon, upon which the heart rests, takes 

 no considerable part in the movement during quiet inspiration; but 

 during forced inspiration it also is depressed to a certain extent. 



In the recumbent posture (especially in men), with full light on the thorax, 

 the contraction of the diaphragm can often be seen directly in the form of 

 like movement beginning in the sixth intercostal space and running downward 

 through from one to three intercostal spaces in accordance with the 

 inspiration. 



The diaphragm undoubtedly plays the most important part in e 

 thorax. Briicke further maintains that the diaphragm, besides enlarg 

 thorax in a vertical direction, also expands the lower part in a transvers 

 tion : namely when it compresses the abdominal organs from above, the 



