784 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



right foot will be gradually drawn up and swept over the point of 

 stimulation to remove the stimulus. The apparently purposeful char- 

 acter of such an action is still more strongly manifested if in such an 

 experiment the right foot be firmly held ; after a few ineffectual con- 

 tractions of the muscles of the right leg the left leg will then be drawn 

 up to remove the offending body. In all cases, therefore, except in those 

 of the very simplest character, the resulting motion produced reflectively 

 from a stimulus is out of all proportion in complexity to the nature of 

 the stimulation. This complexity is much more marked when the 

 stimulus is applied to the terminal corpuscles of sensory nerves, as in 

 the case above alluded to. 



If the stimulation be applied to a sensory nerve-trunk the character 

 of the resultant reflex action is, as a rule, of simpler character ; or, at 

 least, is to a certain extent free from the apparent purposeful character 

 noticed above. Thus, for example, if an induced current be applied to 

 the central end of a divided sciatic nerve general convulsive movements 

 are produced, but no apparently co-ordinate attempt to remove the 

 stimulus can be detected, even with the employment of the weakest cur- 

 rent. It, therefore, is evident that the character of the reflex action 

 depends upon the nature of the locality of application and intensity of 

 the afferent impulses. 



2. AUTOMATISM. By automatism is meant the power possessed by 

 nerve-centres of apparently originating nervous impulses. It is, how- 

 ever, difficult to draw a line between so-called automatic action and 

 reflex action. Thus, the act of respiration, which is a favorable example 

 of the so-called automatic action, is in all probability due to, or, at any 

 rate, largely governed by, the character of the impulses brought to that 

 centre through the various afferent nerves. So, again, the regulation of 

 the calibre of the blood-vessels, which is controlled by the automatic 

 power of the vaso-motor centre, is again largely modified by the nature 

 of the afferent impulses brought to it. 



The clearest example of pure automatic action is to be found in the 

 pulsations of the excised heart. As was seen in the chapter on circula- 

 tion, the heart might be removed from a cold-blooded animal and yet 

 preserve for many hours its power of rhythmical contraction, and it 

 was demonstrated that such automatic action was the result of the 

 function of the nervous ganglia found in the heart. 



So, also, the movements of the alimentary canal were described as 

 of an automatic character, for although their character is influenced by 

 the contents of the intestinal tube, just as the movements of the heart 

 might be influenced by various afferent impulses, the movements of the 

 intestine may occur in an empty condition of the bowel or they may be 

 absent when the canal is filled. In the spinal cord, therefore, centres 



