THE NERVOUS SYSTEM IN ANIMALS. 319 



experiment sufficiently demonstrates in certain cases, that, in animals as in 

 man, even in the simplest forms, in which the nervous system consists of but 

 a single ganglionic centre, the nerve-fibres act internuncially, as mere con- 

 ductors of the effects of impressions produced upon them ; whilst the gray mat- 

 ter, whether aggregated in certain parts of a cerebro-spinal axis, or collected 

 in a small ganglionic mass, is not only a conductor of the effects of impressions, 

 but may transfer, radiate, or reflect those impressions, and may constitute a 

 central sensorium, for the reception of sensory impressions, and, in the higher 

 animals, a centre of origin of motorial stimulus. The gray matter always 

 manifests higher endowments than the nerve-fibres, whether white or gan- 

 glionic. As we have seen, in the study of the human nervous system, some of 

 these fibres must be afferent and some efferent ; certain afferent fibres are con- 

 cerned in the conduction of sensory impressions only, as, for example, the 

 nerves of sight ; whilst others conduct the effects of stimuli to the gray matter, 

 from which a motor influence is reflected upon certain efferent fibres, in reflex 

 actions, either sensori-motor, or simply excito-motor ; whilst lastly, other 

 efferent fibres conduct motorial impressions, which have, according to the 

 position of a given animal in the scale, a more or less distinct ideational, emo- 

 tional, or volitional character. As in man, too, the effects of the volitional 

 stimulus, are of course directed upon the muscles concerned in locomotion ; 

 whilst the involuntary, or reflex, movements partly occur in the locomotive 

 muscular system, and partly, as in man, in the muscular structures concerned 

 in the functions of vegetative life. 



The amount of nervous force manifested by any animal, whether in the 

 phenomena of sensation, or of the regulation of its voluntary and involuntary 

 movements, is strictly in accordance with the relative mass, and complexity 

 of organization, of its nervous centre or centres. It is certain, therefore, that 

 sensation and the power of regulated motion, as well as the higher psychical 

 endowments, by which the animal is governed through trains of ideas, emo- 

 tions, memory, reasoning processes, and will, are more highly developed in the 

 Yertebrata, than in the lower subkingdoms, in which, at length, all sensation 

 and regulation of movement must be reduced to a minimum, and so finally 

 become extinct. As we descend in the scale, the higher psychical endowments 

 first fade away ; the ideational, emotional, reasoning, and volitional faculties 

 disappear, probably not being manifested by any creatures below the Verte- 

 brate type ; and, even in the highest of these, the controlling power of the 

 will over the direction of the thoughts, so peculiarly marked in man, is of 

 doubtful existence. Sensori-motor power, or pure instinct, still persists as the 

 special automatic paramount guiding force, as in insects, for example. In yet 

 lower forms, the movements are probably not even instinctive, but excito- 

 motor or purely reflex ; and lastly, in the very lowest forms, the movements 

 are probably performed by the immediate stimulation of an insensible con- 

 tractile tissue, altogether independent of nervous influence, as in the case of 

 the ciliary motion of the Infusoria, or of the irregular movements of the Amoeba. 

 Even in the highest animals, the ciliary motion, as already stated, appears to 

 be independent of the action of the nervous system. 



Whilst, however, in the preceding general sketch, it is assumed that the 

 nervous system dies out, or disappears, before we reach the lowest confines of 

 animal existence, because no nervous system has been there detected, yet it 

 has been maintained by some, that nervous substance may possibly exist in 

 microscopic ganglia which escape detection ; or that it may be diffused, in the 

 shape of single nerve cells, in the bodies of the minuter organisms ; or lastly, 

 that it may even form a portion of the contents of a uni-cellular animal, that 

 is, of an animal organism consisting but of a single nucleated cell. If these 

 conjectures be true, we must conclude, that no animal exhibits movements, 

 resulting from the direct stimulation of its contractile substances by external 

 agents ; but that, even in the very simplest forms, nerve substance may regu- 

 late those movements. 



The nervous system is an apparatus working so completely in accordance 

 with its structural peculiarities that the successive stages by which its func- 

 tions are gradually simplified may be best followed, and its adaptations to the 

 actions and wants of animals of lower and lower organization be most readily 



