MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
541 
wards, as the proportion of the thoracic to the abdominal viscera increases, a forward 
movement of the lower cartilages takes place ; this gradually increases until, in adult 
life, the forward movement of the conjoint cartilages and of the lower portion of the 
sternum is even greater than that of the superior cartilages and the upper portion 
of the sternum. 
100. Old age*. 
In the aged, the costal cartilages being ossified, each rib and cartilage forms as it 
were one inflexible bone ; the costal cartilages still hinge on the sternum. The costal 
respiration is performed by the elevation and consequent variation in the position of 
the curves of the ribs. There is no yielding of the cartilages, consequently the lower 
part of the sternum and the conjoint costal cartilages are pushed forward to a greater 
extent than they are in the youthful. The thoracic respiration, though considerable, 
is not so great as in the adult or young man ; to make amends, diaphragmatic respi- 
ration is increased, and the angle formed between the opposite costal cartilages below 
the sternum is more open. The old man does not require the energetic breathing of the 
young. The bulk of the lung increases, the air-cells being enlarged, in the aged, and 
the chest gradually takes on more and more the form that it has on an ordinary or 
even a deep inspiration ; consequently the movement of the ribs on inspiration is not 
so great in the old man as it is in the middle-aged. The increase of the dorsal arch 
due to the permanent inspiratory form, causes the stooping, and, in part, the short- 
ening of the aged. 
101. Varieties in respiratory movements induced by disease. 
Permit me to hint at the influences of some diseases of the lungs on the respira- 
tory movements of the chest. 
If the larynx be narrowed so as to permit but little air to enter the lungs, the 
diaphragm descends so rapidly that the air has not time to fill up those portions of 
the lungs displaced downwards by the diaphragm ; the consequence is that the pres- 
sure of the atmosphere forces backwards and inwards the costal walls on the lungs. 
If the summit of the lung be affected with phthisis, the corresponding portion of 
the chest is but little dilated, and the ribs are depressed and almost motionless over 
the diseased portion of lung; sometimes the rib even falls in at the beginning of 
inspiration. 
In inflammation of the lower lobe of the lung, that lobe is distended by diseased 
secretions ; the chest over it is permanently expanded and has little or no respiratory 
movement ; at the same time the upper portion of the same side of the chest is less 
actively inspiratory. 
If the diaphragm be inflamed on one side, that side does not act; the diaphrag- 
matic ribs and the hypogastric region do not move forwards on the affected, though 
they do on the healthy side. 
* Fig. XXIII. a.b. Archives of the Royal Society. 
