CAUSE OF THE RESPIRATORY RHYTHM. 395 



to accumulate, or "gain a head," before they travel out 

 from the centre, and, after their discharge, time is required 

 to attain once more the necessary level of irruption before a 

 fresh innervation is sent to the muscles. This method of 

 accounting for the respiratory rhythm is known as the 

 "resistance theory." If not altogether satisfactory it is at 

 least far preferable to the older mode of considering the 

 question solved by assuming a rhythmic character or prop- 

 erty of the respiratory centre. It gives a definite hypothe- 

 sis, which accords with what is known of general natural 

 laws outside of the Body, and the truth or falsity of which 

 can be tested by experiment: and so serves very well to 

 show how scientific differs from pre-scicntific, or mediaeval, 

 physiology. The latter was content with observing things 

 in the Body and considered it explained a phenomenon 

 when it gave it a name. Now we call a phenomenon ex- 

 plained, when we have found to what general category 

 of natural laws it can be reduced as a special example; 

 and this reducing a special case to a particular manifesta- 

 tion of some one or more general properties of matter 

 already known is, of course, all that we ever mean when we 

 say we explain anything. We explain the fall of an apple 

 and the rise of the tides by referring them to the class of 

 general results of the Law of Gravitation; but the why of 

 the law of gravitation we do not know at all; it is merely a 

 fact which we have found out. So with regard to Physi- 

 ology; we are working scientifically when we try to reduce 

 the activities of the living Body to special instances of 

 mechanical, physical, or chemical laws otherwise known to- 

 us, and unscientifically when we lose sight of that aim. 

 Certain vital phenomena, as those of blood-pressure, we can 

 thus explain, as much as we can explain anything; others, as 

 the rhythm of the respiratory movements, we can provision- 

 ally explain, although not yet certain that our explana- 

 tion is the right one; and still others, as the phenomena of 

 consciousness, we cannot explain at all, and possibly never 

 shall, by referring them to general properties of matter, 

 since they may be properties only of that particular kind of 



