The following experiment proves that the and in the latter they collapse. From 3000 observations we have found that, in deep in spiration, the body becomes more erect, and less so in expiration.
Insufflation is not the same, in effect, as inspiration. In the former we force air into the chest, until the parts most yielding, as the diaphragm and abdominal parietes, are rendered so tense that their tension is suffi cient to overcome the elastic force of the ribs, their cartilages, and the lungs ; then, and not until then, do we move the costal part of the thorax. On the other hand, in the living and deep inspiration, we lift the ribs and sternum, the most unyielding portions, first. These solely produce the threatened vacuum which inflates the lungs, whilst very little, if any, is accomplished by the diaphragm.
The following table shows the measure ments of the thorax, when expanded by in-, spiration and insufflation.
These facts show that we should be guarded in determining the living respiratory move ments by experiments upon the dead subject.
It should be constantly borne in mind, that to increase or diminish an intercostal space is to elevate or depress the ribs, and that to elevate or depress the ribs is to increase or diminish their intercostal spaces : the one cannot be accomplished without the other. Some au thors have spoken of these as distinct; thus, that in inspiration the superior ribs ap proximate each other, whilst they are raised, or that their rising or falling does not neces sarily involve an increase or diminishing of the intercostal spaces ; but these two changes are simultaneous and cannot be done sepa rately.
2nd. The effect of tensions, oblique, perpen dicular, and decussating, between the moveable levers or ribs.—We employ a strip of vul canised Indian rubber for a force representing muscular contraction. A strip of this sub stance, of uniform thickness, of an inch broad and 10 inches long, increased its length, with an increasing weight, as follows :— Although not exactly in accordance with the law of perfect elasticity, yet it is roughly so and enough for our purpose, viz. the tension is greatest when most stretched, and weakest when least stretched, corresponding with muscular contraction.
Let E E (fig. 686.) be fixed, An and c D two moveable bars as before, t an oblique tension ; if t shortens, it has been supposed that the two bars would assume the position of A a' and c n ; but not so : they both rise like A B" and c D" until the two bars touch each other.* If
we prevent this touching of the two bars by a rigid link, like that on parallel rulers, placed as at s fig. 687., then the tension will still raise the bars to o o'. In this experiment three circumstances may be noticed. 1st, that the bars have been elevated ; 2dly, that the perpendicular distance between them has been diminished ; and 3dly, that the tension t has been shortened in attaining the position o o'. Place the tension in a contrary direc tion, as between the bars A' n' and c' n', and the bars are brought into a contrary position, —drawn downwards to o' o'" This can be demonstrated by a model, using a spring or Indian-rubber as the tension, and may be explained as follows. Let A B, fig. 688., sent one bar, the perpendicular fixed body; is the free extremity of the bar ; x an axis from which a parallel bar has been re moved. Let e, i, and o be other fixed points ; connect e to n by an elastic tension, and the bars will be moved towards e. Let the ten sion be fixed at i or o, still the bars will be raised towards the respective points. Let the tension be fixed at x (the centre of mo tion of the bar which we suppose is removed), and still the bar A B will be raised upwards towards x, and assume the position of A B, K L (fig. 689.) at IYI. But it is not necessary to this that the elastic force should extend from otherwise the free bar would approximate the fixed bar c D. Therefore, one fulcrum is pushed upon by one bar, and pulled upon by the other. If the bars were kept fixed, and the body re presentltig the spine was left free, the tension would draw this last mentioned body into the to L in order to produce this motion : half of it might be wood, bone, or iron, provided the other half retained its elastic power. The effect would be the same, and the bar A s at N would be elevated by the tension between T L, position of c c and c' c' Therefore, the element of the two fulcra is the chief agent for directing their upward or downward move ment, under an oblique tension. If we arrange two bars with one fulcrum (fig. 692.), and allow the tension to act as before, then the effect is only to draw the two bars together, as o b and o' d (fig. 693.). If we have an arrange ment to substitute two fulcra at a a' fig. 692. and withdraw the centre fulcrum, then the two bars rise as before.