118 THE MECHANISM OF LIFE 



efferent or motor impulses from the nucleus (or ganglion) to the 

 muscle, and also fibres that convey afferent or sensory impulses 

 from the muscle to the nucleus. Thirdly, the stimulus that 

 starts the nervous impulse and excites the muscular organs to 

 activity nearly always originates in a different segment of the 

 body from that containing the former, so that there must 

 generally be a path, or tract, in the central nervous system itself 

 along which the impulse travels. Lastly, there are hardly ever 

 only two antagonistic muscles concerned in a movement, but 

 rather a muscle system consisting of several or many such pairs. 

 Thus the simplest actually observable action that can occur in 

 the body of a higher animal includes a rather complicated 

 mechanism, and the complexity of this becomes all the greater 

 when we take account of the connections of the nucleus immedi- 

 ately controlling the action with the higher brain centres. When 

 such connections exist we have the possibility that the action 

 may be modifiable to almost any degree by the volition of the 

 animal, or by its " experience." 



Leaving aside, in the meantime, the factors of volition and 

 experience, we may consider the action as it is performed in an 

 automatic, mechanical manner. Let it be that which may occur 

 when the side of the spinal dog* is tickled and the " scratching 

 reflex " occurs (see pp. 119, 137). 



Fig. 34 represents nervous mechanisms that are involved in 

 this action. These mechanisms (to judge from the figure) appear 

 to be somewhat complicated, and yet we have taken account 

 only of those which must be in action, and we have omitted 

 others that are less essential to our present explanation. 



The stimulation, then, of the touch organs in the skin of the 

 body sets up impulses that are transmitted to the spinal cord 

 via the afferent fibres of a spinal nerve. Now this segment of 

 the cord is well in front of that from which are given off the 

 motor nerves supplying the hind-limb, and so there must be a 

 nervous tract connecting centres (a) and (&). The afferent 

 impulse, after being received by the synapses of the grey matter 

 in centre (a), is modified in some way, is retransmitted along 

 a tract of fibres in the white matter of the cord, and is received 

 by motor nerve cells in segment (6). We must think about 

 these motor nerve cells as being arranged in some way or other 



* A " spinal animal " is one in which the brain has either been destroj'cd 

 altogether (by operation) or has been separated from the spinal cord. 

 Thus the muscles of the body are entirely controlled by the cord. 



