CONDITIONS OF NERVOUS ACTIONS. 229 



movement. We shall hereafter see, that nearly all those movements in 

 the animal body, which are immediately connected with the mainte- 

 nance of the organic functions, such as those of respiration, degluti- 

 tion (or swallowing), the expulsion of the faeces, urine, and foetus, &o. 

 are performed in this manner. 



395. Now there is strong reason to believe that the changes which 

 take place in the nervous trunks are of the same nature, whatever may 

 be the source from which they proceed, ^whether, for example, the 

 movement is simply reflex, whether it proceed from a mental emotion, 

 or whether it be executed in obedience to an act of the will. It was 

 formerly supposed that all. the afferent or centripetal fibres pass up to 

 the Brain, and that all the efferent or centrifugal fibres pass down from 

 the same organ ; the Spinal Cord being looked upon as little else than 

 a bundle of nerves. It is now known, however, that by far the greater 

 part of the fibres of any trunk terminates in the central organ, to which 

 that trunk at first proceeds ; and that the Spinal Cord may be consi- 

 dered as a series of such ganglionic centres, each receiving the afferent 

 fibres, and giving origin to the efferent, of its own segment. So, 

 again, the special sensory nerves, the olfactive, optic, auditory, and 

 gustative, terminate in their own ganglionic centres, which lie at the 

 base of the brain, in immediate connexion with the summit of the 

 spinal cord, and which are quite independent of the cerebrum. The 

 apparatus for receiving impressions, and for originating motions,. is thus 

 complete in itself; and the addition of the cerebrum does not make 

 any essential difference in its operations, save that this sensori-motor 

 apparatus (as it may be termed) is made to act through its means as 

 the agent of the mind, in addition to its functions as the instrument of 

 the automatic movements. We shall hereafter see (CHAP, xn.), that 

 the difference between Instinct and Intelligence is closely connected 

 with the development of the cerebrum ; but that this organ, even in 

 that highest grade of development which it possesses in Man, has no 

 other connexion with the sensory organs than that which it acquires 

 through its relation with the sensory ganglia, and has no more power of 

 exciting muscular movement, than by playing (so to speak), upon the 

 spinal cord, whose efferent fibres respond to its mandates, just as they 

 would do to the stimulus of an impression primarily acting through that 

 organ. 



396. Of the mode by which the effects of changes ^in one part of the 

 Nervous system, are thus instantaneously transmitted to another, 

 nothing whatever is known. There is evidently a strong analogy between 

 this phenomenon, and the instantaneous transmission of the Electric 

 power along good conductors ; but the relation is much more intimate 

 than this, for Electricity is capable of exciting Nerve-force, whilst, 

 conversely, Nerve-force can excite Electricity. Thus, a very feeble 

 galvanic current transmitted along a motor nerve, serves to excite con- 

 tractions in the muscles supplied by it ; and in like manner, a galvanic 

 current transmitted along any of the sensory nerves, gives rise to a 

 sensation of the kind to which the nerve ministers. Moreover we 

 shall hereafter see, that certain animals are capable of generating 

 Electric power in a very remarkable manner (CHAP, x.) ; and that the 



