82 NERVOUS SYSTEM OF VERTEBRATES. 



by all animals may be examined. If a frog be touched or pinched 

 it will usually make a single prompt movement which may result 

 in drawing the part concerned out of danger. If a flash of light 

 suddenly falls on the eye of a fish, or if a swiftly moving shadow 

 crosses the water, the fish may make a quick dart due to a single 

 stroke or a few strokes of the tail. Further movements of either 

 frog or fish may be indirectly connected with the stimulus men- 

 tioned, but it is the initial, relatively simple movement which 

 concerns us here. We ourselves make similar movements 

 under certain conditions. In our sleep we may move a hand 

 to brush away a fly without being conscious of the act. We 

 commonly toss about more or less in our sleep, and quite uncon- 

 sciously. Certain waking movements are also of the same simple 

 class, as when the eye-lid is suddenly closed to shut out a flying 

 insect which is not consciously seen, or when sudden coughing 

 is caused by some object entering the trachea. Probably many 

 simpler mechanical operations, such as walking, fall at least at 

 times under the same categoiy of simple actions. 



If these simple actions are studied experimentally under proper 

 conditions we can determine what nen-e elements are engaged and 

 how they act. First, if in a frog or other lower animal the brain 

 be entirely destroyed, the sort of actions mentioned are still per- 

 formed with normal efficiency. Only the spinal cord and the 

 peripheral elements connected with it are necessary. In the 

 simplest case a single receptive cell whose body is situated in the 

 spinal ganglion, receives the stimulus through the terminal branches 

 of its dendrite in the skin and transmits an impulse along its cen- 

 trally directed neurite. Within the spinal cord lateral branches 

 are given off from the neurite and one of these collaterals carries 

 the impulse to one or more excitatorv^ cells lying in the ventral 

 horn of the cord. These cells send impulses out along their neu- 

 rites in the xentral root of a spinal nerve to certain muscles whose 

 contraction produces the observed movement. Thus only recep- 

 tive and excitatory cells are engaged in the whole action from the 

 reception of the stimulus to the movement in response. This is 

 iUustrated in the left half of Fig. 41, A. The movement is called 

 a reflex movement, the whole act including the nervous processes 



