378 OF RESPIRATION. 



an unusual enlargement of the upper part of the thoracic cavity. When 

 the Expiratory action is to be performed, the descent of the ribs is 

 occasioned by the muscles of the spine and abdomen, which proceed 

 upwards from the lower part of the trunk ; and this action is aided by 

 the elasticity of the costal cartilages. 



681. In the ordinary act of inspiration, however, the Diaphragm 

 performs the most important part. The contraction of this muscle 

 changes its upper surface, from the high arch that it forms when 

 relaxed and pushed upwards by the viscera below, to a much more level 

 state ; though it never approaches very closely to a plane ; being some- 

 what convex, even when the fullest inspiration has been taken. When 

 thus drawn down, it presses upon the abdominal viscera, and causes 

 them to project forwards, which they are allowed to do, by the relaxa- 

 tion of the abdominal muscles. In tranquil breathing, this action is 

 alone nearly sufficient to produce the requisite enlargement of the tho- 

 racic cavity ; the position of the ribs being very little altered. In the 

 expiratory movement, the diaphragm is altogether passive ; for, being 

 in a state of relaxation, it is forced upwards by the abdominal viscera, 

 which are pressed inwards by the contraction of the abdominal muscles. 

 These last, therefore, are the main instruments of the expiratory move- 

 ment; diminishing the cavity of the chest by elevating its floor, 

 at the same time that they draw its bony framework into a narrower 

 compass. 



682. In this manner, by the regularly-alternating dilatation and con- 

 traction of the thoracic cavity, the air within the lungs is alternately in- 

 creased and diminished in amount ; and thus a regular exchange is 

 secured. This exchange, however, can only affect at any one time a 

 certain proportion of the air in the lungs ; thus it is probable, that the 

 quantity remaining in these organs after ordinary expiration is above 

 100 cubic inches, whilst the amount usually expired is not above 20 

 cubic inches. Indeed if it were not for the tendency of gases to mutual 

 diffusion, the air in the remote air-cells might never be renewed. If 

 any aperture exist, by which air could obtain direct access to the pleural 

 cavity, the lungs would not be dilated by its enlargement ; for the 

 vacuum would be supplied much more readily, by the direct ingress of 

 the air (provided the aperture be large enough), than by the distension 

 of the lung. Thus a large penetrating wound of the thoracic cavity 

 may completely throw out of use the lung of that side ; and the same 

 result will follow, when an aperture forms by ulceration in the sub- 

 stance of the lung itself, establishing a free communication between the 

 pleural cavity and one of the bronchial tubes ; so that, of the air which 

 rushes in by the trachea, to compensate for the enlargement of the thoracic 

 cavity, a great part goes at once into that cavity, without contributing 

 to the distension of the lungs, and therefore without serving for the 

 aeration of the blood. 



683. The number of the respiratory movements (that is, of the acts 

 of inspiration and expiration taken together) may be probably estimated 

 at from 14 to 18 per minute, in a state of health, and of repose of body 

 and mind. Of these, the greater part are moderate in amount, involv- 

 ing little movement except in the diaphragm ; but a greater exertion, 



