THE RELATIVITY OF ALL KNOWLEDGE. 73 



concerning organic actions, which, conspicuous and remark- 

 able as they are, you had not before cared to understand. 

 How is respiration effected? you ask — why does air periodi- 

 cally rush into the lungs? The answer is that in the higher 

 vertebrata, as in ourselves, influx of air is caused by an en- 

 largement of the thoracia cavity, due, partly to depression 

 of the diaphragm, partly to elevation of the ribs. But how 

 does elevation of the ribs enlarge the cavity? In reply the 

 anatomist shows you that the plane of each pair of ribs 

 makes an acute angle with the spine ; that this angle widens 

 when the movable ends of the ribs are raised ; and he makes 

 you realize the consequent dilatation of the cavity, by point- 

 ing out how the area of a parallelogram increases as its an- 

 gles approach to right angles — you understand this special 

 fact when you see it to be an instance of a general geometri- 

 cal fact. There still arises, however, the question — why 

 does the air rush into this enlarged cavity ? To which comes 

 the answer that, when the thoracic cavity is enlarged, the 

 contained air, partially relieved from pressure, expands, and 

 so loses some of its resisting power; that hence it opposes to 

 the pressure of the external air a less pressure; and that as 

 air, like every other fluid, presses equally in all directions, 

 motion must result along any line in which the resistance is 

 less than elsewhere; whence follows an inward current. 

 And this interpretation you recognize as one, when a few 

 facts of like kind, exhibited more plainly in a visible fluid 

 such as water, are cited in illustration. Again, when 



it was pointed out that the limbs are compound levers acting 

 in essentially the same way as levers of iron or wood, you 

 might consider yourself as having obtained a partial ration- 

 ale of animal movements. The contraction of a muscle, 

 seeming before utterly unaccountable, would seem less un- 

 accountable were you shown how, by a galvanic current, a 

 series of soft iron magnets could be made to shorten itself, 

 through the attraction of each magnet for its neighbours: 

 — an alleged analogy which especially answers the pur- 

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