MR. S1BS0N ON THE MECHANISM OF RESPIRATION. 
539 
extent. The central tendon in its descent stretches and elongates the pericardial sac, 
increasing the space containing the heart. When the diaphragm is forced up by the 
contraction of the abdominal muscles, the tendinous fibres with which it invests the 
vena cava, interstitially support the mass of the liver. 
96-100. Varieties in the mechanism of respiration in the human subject due to age 
and sex. 
96. In the human subject the extent of the lung varies with the age and sex of 
the individual. In the adult well-formed man, the chest is apparently of greater 
bulk than the abdomen. The proportion of the chest to the abdomen is greater in 
the active than the inactive. 
97. Female*. 
In the female the development of the abdomen in relation to the chest is consider- 
ably greater than it is in man. Owing to the tightness with which the stays are 
usually worn round the lower or diaphragmatic portion of the chest, the diaphragm 
is compressed and its inspiratory expansion is greatly impeded. The opposite con- 
joint cartilages of the sixth, seventh and eighth ribs (the intermediate set) are pressed 
nearer to each other, so that instead of forming an angle with each other, they become 
almost parallel. To make amends for the artificial impediment to respiration of the 
diaphragmatic portion of the lungs, the thoracic portion is unnaturally developed. 
98. Foetus' and new-born infant 
In the child §, and still more in the foetus and new-born infant, the abdomen and its 
viscera are very large in proportion to the chest and its contents. While the lungs 
are small, the liver is of great size. 
In the foetus, the unexpanded lungs occupy a very small space ; the sternum and 
costal cartilages fall inwards and form a hollow ; the angle between the opposed 
sixth, seventh and eighth costal cartilages below the sternum is obtuse, and the 
abdominal viscera push outwards the inferior ribs to a very great extent. 
In the new-born infant the sternum and costal cartilages, by virtue of their elas- 
ticity and of muscular actions, move forwards, and from being concave become flat 
or slightly convex ; the sixth, seventh and eighth cartilages form with each other a 
right angle. 
Influence of the elasticity of the walls of the chest on the first inspiration and on 
respiration. 
The inspiratory expansion of the chest in the new-born infant illustrates well the 
effect of the elasticity of the thoracic walls in maintaining a considerable portion of 
* Fig. XIX. Archives of the Royal Society. t Fig. XXIV. Archives of the Royal Society. 
t Fig. XXV. Ibid. § Fig. XXI. XXII. Ibid. 
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