MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 227 



aperture. When the diaphragm contracts, even in ordinary quiet 

 breathing, the central tendon descends distinctly (about half an 

 inch) after the manner of a piston. The acute angle which the 

 muscular ring makes during relaxation with the thoracic wall opens 

 out around its whole circumference, so as to form a groove of trian- 

 gular section. But the most peripheral portion of the ring is always 

 kept in close apposition to the chest-wall by the negative intra- 

 thoracie pressure. The lungs follow the descending diaphragm, 

 their lower borders keeping accurately in contact with it. The 

 descent of the diaphragm is not directly downwards, but downwards 

 and forwards. For it is compounded of two movements, the spinal 

 segment of the muscle (the crura) causing a vertical elongation of 

 the thorax, while the sterno-costal part (the muscular ring) pushes 

 the abdominal viscera downwards and forwards (Keith). Since 

 the diaphragm is attached to the lower ribs, there is a tendency 

 during its contraction for these to be drawn inwards and upwards; 

 but this is opposed by the pressure of the abdominal viscera, and by 

 the action of the quadratics lumborum, which fixes the twelfth rib, 

 and of the serratus posticus inferior, which draws the lower four 

 ribs backward. When these and the other inspiratory muscles 

 that act especially upon the ribs are paralyzed by injury to the 

 spinal cord, and respiration is carried on by the diaphragm alone, 

 the line of its attachment to the ribs is distinctly marked during 

 inspiration by a shallow circular groove. 



The thorax is also enlarged by the action of certain muscles that 

 act upon the ribs. Among the elevators of the ribs, as their name 

 indicates, are usually reckoned, although erroneously, the levatores 

 costarum twelve in number on each side. They arise from the 

 transverse processes of the last cervical and first eleven dorsal 

 vertebrae, and passing obliquely downwards and outwards, are in- 

 serted between the tubercle and the angle into the first or second rib 

 below their origin. They do not elevate the ribs, but take part in 

 lateral movements of the spinal column. The scalene muscles, 

 which may in a lean person be felt to be tense during inspiration, 

 fix the first and second ribs (scalenus anticus and medius, the first ; 

 scalenus posticus, the second rib), and so afford a fixed line for the 

 intercostal muscles to work from on the lower ribs. 



The most important elevators of the ribs are the external inter- 

 costals. The intercartilaginous portions of the internal inter- 

 costals (the intercartilaginei muscles, as they are sometimes called) 

 also contract simultaneously with the diaphragm, and may there- 

 fore be included in the list of inspiratory muscles; but instead of 

 elevating the ribs they depress the costal cartilages, and 'thus help 

 to widen the angles between them and the ribs. In addition to 

 increasing the capacity of the chest, the contraction of the external 

 intercostals and the intercartilaginous muscles aids in inspiration 



