THE RESPIRATORY MECHANISM. 419 



nervation is sent to the muscles. This method of account- 

 ing 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 property of the 

 respiratory centre. It gives a definite hypothesis, which 

 accords with what is known of general natural laws outside 

 of the Body, and the validity of which can be subjected to 

 experiment: and so serves very well to show how scientific 

 differs from pre-scientific, 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 explained, 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 manifestation 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 Physiology; 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 ex- 

 plain anything; others, as the rhythm of the respiratory 

 movements, we can provisionally explain, although not yet 

 certain that our explanation 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 associated only with 

 that particular kind of matter called protoplasm, and per- 

 haps only with some varieties of it. 



The Relation of the Pneumogastric Nerves to the Re- 

 spiratory Centre. We have next to consider if any phenom- 

 ena presented by the living Body give support to the resist- 

 ance theory of the respiratory rhythm. A very important 

 collateral prop to it is given by the relation of the pneu mo- 

 gastric nerves to the rate and force of the respiratory move- 





