RESPIRATION 211 



chest falls back to its original limits ; the pressure in the thoracic 

 cavity increases ; the distended lungs, in virtue of their elasticity, 

 shrink to their former volume ; the pressure of the air in the 

 alveoli rises above that of the atmosphere, and with this reversal 

 of the slope of pressure air streams out of the bronchi and trachea. 



In inspiration the chest dilates in all its diameters. Its 

 vertical diameter is increased by the contraction of the 

 diaphragm, which, composed of a central tendon, a peripheral 

 ring of muscular tissue, and the two muscular crura, bulges up 

 into the thorax in the form of two flattened domes, one on each 

 side, and thus closes its lower 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 triangular section. But 

 the most peripheral portion of the ring is always kept in close 

 apposition to the chest-wall by the negative intrathoracic 

 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 quadratus 

 lumbomm, 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 inserted 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 



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