126 AN INTRODUCTION TO SCIENCE 



organs are not equally endowed with nerves. ' ' Since in the 

 course of evolution a universal development of motor nerves 

 has not occurred, it is, I think, to be expected that the de- 

 velopment of inhibitory fibres should be still less universal." 

 Our knowledge of the anatomy, chemistry and physiology 

 of the nervous system has enormously increased in the last 

 few years, but our conceptions of its relation to the several 

 organs and tissues of the body are far less precise than they 

 used to be. It is another illustration of the tendency towards 

 agnosticism, and a hopeful sign of the suppression of the 

 besetting sin of the physiologist, who expects to find every 

 part of the body's work being done as he would order it in 

 his house or factory or laboratory. 



As a last illustration of the direction in which physiologi- 

 cal thought is trending, we may point out that the doctrine 

 of the minute subdivision of functions among the constitu- 

 ent parts of the grey matter of the central nervous system 

 has been rudely shaken of late. We have but space for 

 three examples. And first, if the reader will place his finger 

 upon his pulse when he drinks a glass of water, he will find 

 that while he is drinking the heart beats more quickly. 

 This shows that the impulses which pass through the 

 medulla- oblongata and lead to the inhibition of the heart 

 are checked during the reflex action of swallowing. The 

 carrying out of one reflex is therefore associated with the 

 blocking of other reflex paths. Secondly, if the reflex 

 action or pseudo-reflex action known as the knee-jerk is 

 properly investigated, it is found that the passage of the 

 nerve-current which leads to this action is affected by 

 events which are occurring in far-distant parts of the central 

 nervous system. If, when a person is sitting with his knees 

 crossed and the foot hanging free, the tendon below the 

 knee-cap is tapped, say with a paper-knife or other blunt 

 object, the foot is jerked out without any regard for its 

 owner's wishes. It is possible so to arrange matters that 

 the tendon is tapped with a hammer worked by clockwork 



