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MANUAL OF PHYSIOLOGY. 



Those which are strictly inspiratory in their function are : 



1. The Diaphragm with its accessory Quadratus Lumborum 



to fix its origin from the last rib. 



2. Levatores costarum (including the scaleni) with their 



accessory intercostals, which act chiefly as regulators. 



3. The Serratus posticus superior. 



The Diaphragm is the most important inspiratory muscle. It 

 is the only one muscle which unaided can keep up the necessary 

 thoracic ventilation, and, in injury of the spinal cord, owing to 

 its isolated nervous supply, it may be called upon to do so. 



FIG. 150. 



Diagram of a section made vertically from side to side through the thoracic and part 

 of the abdominal cavities to show the position of the diaphragm, which is indicated by 

 the dark line (D D) placed on the parts of the muscle that descend in inspiration. p. 

 Pericardial cavity. L. Liver, s. Stomach. R. Roots of lungs cut through. 



During ordinary quiet breathing in the male it does the greater 

 part of the work. 



When not in action, a great part of the muscular sheets of the 

 diaphragm lies in direct contact with the inner surface of the 

 lower costal part of the thoracic wall, the rest is higher than the 

 central tendon that forms the floor of the pericardium and is 

 fixed in one position. During inspiration these lateral parts are 

 separated from the ribs and drawn below the level of the central 

 tendon by the contraction of the muscular fibres. The separa- 



