538 
MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
Mr. Hutchinson, whose ingenious inquiries have thrown much light on the mecha- 
nism of breathing. 
In every act of violent exertion, in lifting weights, in coughing, laughing and cry- 
ing, in the violent convulsive fits of epilepsy, and in many other energetic acts, the 
vocal chords are forcibly closed by muscular actions ; the air cannot be expired, and 
the expiratory muscles all combine in the attempt to force out the air from the lungs, 
but ineffectually till the vocal chords are separated. The muscles of the limbs have 
an admirable fixed point for the centre of their actions, when the chest is rigid with 
this violent but ineffectual straining at an expiration. 
95. The diaphragm. 
It is not my intention in this paper to describe minutely the form and respiratory 
actions of the diaphragm ; but this muscle must not be passed over, as it plays so 
important a part in the breathing machinery, and modifies so much the form, the 
number and the movements of the lower set of ribs, or those that assist in diaphrag- 
matic respiration. 
I have seen the action of the diaphragm in the Dog, the Ass, the Rabbit, and the 
domestic Fowl. The form and position of the diaphragm depend entirely on 
the relative proportion of the lower and posterior, or diaphragmatic, to the upper and 
anterior, or thoracic portion of the lung. 
In the Ass, the upper or thoracic portion of lung is very narrow and has but 
little play ; the diaphragmatic portion is of great bulk and expansibility posteriorly ; 
there is but a small extent of lung anteriorly. The diaphragm is very oblique, tend- 
ing to the vertical direction ; its anterior fibres are strong but short ; its lateral fibres 
are long; they are inserted into an oblong central tendon. When the diaphragm 
acts, the diaphragmatic portion of the chest is expanded behind and to the side. The 
body and lower or posterior portion of the diaphragm moves forwards and down- 
wards to a great extent, the anterior portion descends very little, and the whole 
muscle becomes comparatively flattened, § 35. 
In the Dog, the upper or thoracic portion of the lungs is much expanded on in- 
spiration ; the lower surfaces of the lungs and the diaphragm are almost horizontally 
concave. The diaphragm has long muscular fibres anteriorly ; its descent being 
almost as great anteriorly as posteriorly, § 35. 
In Man, the diaphragm is oblique, approaching the horizontal direction. The right 
side is larger and bulges higher into the chest than the left. From the floor of the 
pericardial sac is given off a strong tendinous web that sheaths the whole pericardium 
and is inserted into the investments of the great vessels at the upper part of the 
chest. A thin aponeurosis ascends over each pleura, another descends on the peri- 
toneum. A strong tendinous sheath rises from the central tendon, surrounds the 
vena cava hepatica, and invests all its branches. On inspiration, the bulges of the 
diaphragm are first flattened, and then the whole muscle descends to a nearly equal 
