226 



ELEMENTS OF SCIENCE 



When the influence originated by touching, pricking, 

 or burning some part e.g., some part of the leg is pre- 

 vented by any cause from reaching the brain, while 

 none the less some appropriate action follows e.g., the 

 withdrawal of a foot from a hot iron, without the occur- 

 rence of any sensation such response is called reflex 

 action. The unfelt influence travelling upwards and 

 inwards is supposed, on reaching the spinal cord through 

 the posterior roots of its nerves, to be there auto- 

 matically reflected outwards, through their anterior roots, 



FIG. 44. 



A '1~~ A 



TRANSVERSE SECTION OF THE SPINAL CORD AND ROOTS OF THE 

 SPINAL NERVES OF A CAT. 



A Anterior roots. 

 P Posterior roots. 

 U Compound nerve formed by the 



junction of these roots. 

 b Branches given forth from the 



united nerve. 



1 Anterior (or ventral) median 



fissure of spinal cord. 



2 Posterior (or dorsal) median 



fissure. 



to the nervous fibres which pass to the muscles and 

 excite motion in them. 



Bnt a response quite independent of volition may also 

 take place when feeling is in no way impaired. Thus, if 

 a small object be placed sufficiently far back in the 

 mouth, the muscular act of swallowing will be performed 

 automatically. 



It is evident, as before said, that the cat can feel 

 pleasure and pain, and can experience a variety of 

 definite sensations (of sight, sound, odour, <fec.), which 



