22 



HABIT AND INTELLIGENCE. 



[chap. 



their rela- 

 tion to the 

 sensory 

 ganglia. 



Consen- 

 sual 

 action : 



its simi- 

 larity to 

 merely 

 reflex 

 action. 



Sensation 

 at first is 

 only the 

 guide to 

 action. 



motion is not merely reflex, but determined by sensation 

 and will. The relation of these to the nerves and ganglia 

 of sense is as follows : — An impression of sense is trans- 

 mitted by some of the nerve-fibres of that sense to its 

 ganglia, and the reception of the impression by the ganglia 

 produces the sensation which is appropriate to that im- 

 pression: the sensory ganglia, in their turn, act on the 

 corpora striata, which are motor ganglia; and the latter 

 send down the motor nerves whatever motor impulse is 

 necessary in order to make the appropriate response to the 

 sensation. !For instance : a flash of light falls on the 

 retina, and the impression is telegraphed by the optic nerve 

 to the optic ganglia, where it produces the sensation of 

 light; the optic ganglia act on the corpora striata, and 

 cause them to send a motor impulse to the muscles of the 

 eyelids, which closes the eyes, and thus makes the appro- 

 priate response to the impression of the flash of light. 

 This is what Dr. Carpenter calls consensual action. The 

 chain of cause and effect is exactly the same in consensual 

 action as in merely reflex, except that in consensual action 

 one of the links of the chain is sentient, and the motor 

 action will not be produced unless sensation is felt. Both 

 reflex and consensual action consist simply in, first, an 

 impression sent from the exterior extremity of a nerve to 

 the nervous centre ; and, second, a motor impidse sent 

 outwards from the nervous centre in response to the 

 impression. 



It will be observed that in the foregoing account of 

 consensual action, sensation is described as existing, not by 

 itself, but only as the intermediate link between impres- 

 sions received by the organism from without, and the mus- 

 cular actions that constitute the appropriate response to 

 those impressions. Sensation existing by itself, and not 

 necessarily leading to action, appears to belong to a liigher 

 development of life, and to be the preparation for Mind. 

 An insect may no doubt have sensations without making 

 any motion in response to them : if such sensations exist, 

 we can have no evidence of their existence. But we know 

 that the first and lowest functions of the nervous system 



