THE NERVOUS SYSTEM 331 



animal's body. The conception of definite paths, to which 

 the contemplation of permanent reflexes gives rise, is in- 

 appropriate to occasional reflexes. The latter show so wide a 

 range of variability and adaptability as to prove that a given 

 receptor may bring any of a great variety of groups of motor 

 neurones into connection with itself ; just as a given group of 

 neurones may be played upon by impulses from a great number 

 of different receptors. We have called it a tuning of the 

 motor neurones. One metaphor is as good as another. The 

 physical process which in the brainless frog underlies the 

 preparation for discharging motor neurones in the spinal cord, 

 on the same side as the leg on which vinegar is placed, so long 

 as that leg is free, and on the opposite side, when that leg is 

 fixed, is unknown. We seem to catch a glimpse of a double- 

 ness of action, receptors in the muscles combining with re- 

 ceptors in the skin in determining the paths along which im- 

 pulses shall be reflected the efficient muscles sensitizing their 

 own neurones to the tuning influence of impulses from sounding 

 cutaneous nerve-endings. But it is impossible to formulate a 

 working scheme in the present state of knowledge. 



Sense-Organs and Nerve-Centres. A vast amount of labour 

 has been devoted to the study of the external form of the 

 central nervous system and to unravelling its internal structure ; 

 to plotting out its various groups of nerve-cells, to disentangling 

 its innumerable tracts of fibres. The surface of the brain and 

 spinal cord has been mapped and measured. Every milli- 

 metre of its substance has been cut into sections on the micro- 

 tome. Organs which, fifty years ago, appeared too complicated 

 for investigation have been described in the minutest detail. 

 An immense accumulation of data is available for purposes of 

 reference ; yet anyone who submits the theory of the nervous 

 system as it is held at the present day to a general review must 

 allow that the results of anatomical research enter but little 

 into its construction. The reason for this is not far to seek. 

 As knowledge has advanced, the apparent, or rather the ex- 

 pected, complication of the system has given place to ideas of 

 unity and simplicity. Its external configuration and the varied 

 arrangement in " nuclei " of its nerve-cells may, without im- 

 propriety, be described as accidental. The form of the body 

 and the consequent location of the clients of the nervous 



