the same power as tensions P which is much nearer to the fulcra A B.
Knowing now the effect of a single oblique tension, it is easy to consider an indefinite number of tensions, for they follow the same law of action.
Infig.699. the tensions x acting on A C, In deep inspiration it will be found that the ribs increase the breadth of their intercostal spaces (as was mentioned years ago by Ham berger) ; and that by the contraction of an intercostal fibre. The bars (fig. 698.) in ro tating, twice attain a minimum, and once a maximum, distance from each other; while the oblique tension in that revolution once attains its maximum, and once its minimum length.
Of tensions at djferent parts of the bars or ribs.— Parallel tensions of equal power pro duce the same effect, whether near the fill crum or more distant from it ; an intercostal fibre near the vertebrae, has the same power as a fibre near the anterior extremity of the ribs.
Let Fig. 696. represent ribs as before, with two parallel tensions at different distances from the body, E E, then t' +A M-t' +a mr=e+.r (A a vt)=e+ A t + A N-- n= t-F(A N-a n)= t + A O.
Therefore the tension L T (fig. 694.) acts with the bars to m', and, by the same reason, ten tions observing a contrary direction and acting upon At c' lift the bars to m"'. Therefore tensions, although observing contrary direc tions, may be made to conspire to the same action, and may therefore be associates when acting upon different fulcra. We have shown that tensions in contrary directions, but acting upon the same fulcra, are antagonists. If we join the levers ( fig. 699.) and increase their number, we may represent the thorax as in fig. 700., k k representing the spine, b b the sternum, the bars a a the ribs, and the bars a' a' the costal cartilages united to the bars representing ribs by a movable joint ; let r represent the external intercostals, these, we know, will act as elevators, while those at h, representing the internal intercostals, are asso ciates in action, although they observe a con trary direction, because they act upon the fulcra in b b; in fact, they are elevators of the levers representing the cartilages.
What now is the combined action of a series of two such tensions ? The whole body of levers will be raised, and the part bb re presenting the sternum will have two motions : it will be raised and moved forwards into the position of b' b'. This is precisely the mo
tion of the sternum in deep inspiration. In a model of this kind, certain means must be used to limit the motion, or the movement is continued until the tensions are at rest. Or, if the bars representing the ribs a a (fig. 700.) be fixed, then the tensions representing the internal intercostals h would depress the short bars representing the car tilages, because b b is free, and k k is fixed. And were either set of tensions continued over the joints representing the union of the cartilages with the ribs, such fibres would antagonise each other. In nature the external intercostals are not continued over to the in tercartilaginous spaces.
In this model all the ribs are elevated with out the first bar being drawn up by any ten sion representing the scalenii muscles. In fact the fulcra are the fixed points ; therefore there are here six fixed points for the tensions to act upon, and hence each row of oblique tensions acts quite independently.
This application of force and disposition of the bars representing the ribs is after the manner of that great engine of power the " genou lever." (Vide Potter's Mechanics.) Tensions in the first space (fig. 700.) act with a greater power in pushing out the body b than do the tensions in the 5th space ; but, on the other hand, the motion is less in the 1st than in the 5th space. It will also be seen that these six bars, though moving all equally, will produce an unequal effect upon the body b b, forcing out the lower end more than the upper end. The ribs of man, in the same manner, increase in their length from the 1st to the 8th, and, therefore, by an equal mobility, an unequal protrusion of the sternum is produced, advancing the lower end more than the upper end. 'We have reason to believe that the mobility of all the ribs is the same, and that it is by their different lengths that the different degrees of protrusion of the anterior part of the thorax may be accounted for.