262 NATURE AND MAN. 



difference, each serving, like a telegraph-wire, to convey "mole- 

 cular motion " (the now fashionable mode of expressing a change 

 of whose nature we really know nothing whatever) in either 

 direction, and its function depending entirely upon its connec- 

 tions. The subsequent progress of inquiry, moreover, has made 

 it clear that such " molecular motion," transmitted from a recipient 

 organ to a nerve-centre, may there excite a motor response with- 

 out any affection of the consciousness ; and hence the " sensory " 

 nerves of Bell are now more generally termed " afferent," or 

 "centripetal." 



The " nervous circle," as it was termed by Bell, composed of 

 a sensory nerve, the nerve-centre to which it proceeds, and the 

 motor nerve passing forth from that centre to the muscles, was 

 distinctly recognized by him as furnishing the mechanism of 

 those involuntary movements which are called forth by sensory 

 impressions ; as when the passage of a crumb of bread, a drop 

 of water, or a whiff of acrid vapour into the larynx, excites the 

 act of coughing; — the impression transmitted upwards by the 

 sensory nerves, to a certain part of the brain (including in this term, 

 for the present, the whole aggregate of nerve-centres contained 

 in the cranial cavity), making itself felt there, and calling forth, 

 through the motor nerves that proceed to the muscles of expira- 

 tion, a combined movement adapted to get rid of the source of 

 irritation. This is a typical example of what is now termed reflex 

 action which may be regarded as the elementary form of nervous 

 activity. 



In such a low and almost homogeneous organism as that of 

 the hydra (or fresh-water polype), however, every part seems 

 equally capable of receiving impressions and of responding to 

 them by contraction. As there are neither special sense-organs 

 nor special muscles, there are no special nerves ; and the move- 

 ments by which it grasps the prey that may come within its reach, 

 and draws it into its digestive cavity, are no more indicative of 

 consciousness or will, than are those of the muscles of the gullet 

 that carry down into the stomach the food which is brought within 

 their grasp in the act of swallowing, or than the churning action 

 of the stomach itself during the process of digestion. The con- 

 tinuance of these movements in the alimentary canal of higher 



