FUNCTIONS OF NERVOUS TISSUE. 349 



subdivides, and which comes into intimate relation with the elements of the 

 Muscular tissue. Such an apparatus, altogether designated as the "nervous 

 circle," seems to constitute the entire Nervous System of some among the lower 

 animals ; and it may be multiplied to any extent, so as to supply a great number 

 of similar organs (as we see in the circular gangliated cord surrounding the 

 mouth of the Star-fish, and the double gangliated column which extends along 

 the body of the Worm or the Centipede), without any essential change in its 

 endowments. And even in Man, we shall hereafter find that a considerable part 

 of the Nervous system is constructed upon this simple plan ; although its uni- 

 formity is obscured by the frequent coalescence of ganglionic centres that remain 

 distinct in the lower animals, and by the greater variety in the distribution of 

 the nerve-trunks, in accordance with the dissimilarity between the several organs 

 with which they are connected. The modus operandi of such a system is as 

 follows. By contact, pressure, or some other form of mechanical agency, an 

 impression is made upon the peripheral extremities of the afferent nerves; and 

 this impression, or rather, the change induced by it in the condition of the 

 nerve-fibre, is transmitted by the nerve-trunk to the central ganglion. In this 

 ganglion, the influence transmitted by the afferent trunk excites a reactive change; 

 the occurrence of which is indicated by the transmission) along the efferent 

 nerves, of an influence, which, being distributed to the muscular substance, 

 excites it to contraction. All this takes place so instantaneously, that the move- 

 ment follows immediately upon the application of the stimulus. Actions of this 

 kind, which do not involve Sensation as one of their conditions, and which are 

 executed in direct and unconscious respondence to external stimuli, are now 

 generally termed "reflex;" though the designation " exci to-motor" (originally 

 proposed by Dr. M. Hall) would perhaps be more appropriate, as distinguishing 

 this class of actions from others which have an equal claim to the term " reflex." 

 351. Such operations as the foregoing, however, must be considered as consti- 

 tuting the lowest kind of functional activity of the Nervous system; since they 

 serve only an internuncial purpose, that of bringing the different parts of the 

 bodily organism into co-operative relationship with each other. A much higher 

 attribute is that by which it brings the conscious Mind into relationship with 

 the body, and, through its medium, with the external world; informing it, 

 through the impressions received by the organs of sensation, of the changes 

 which the material objects around it undergo; and enabling it to react upon 

 these, by the instrumentality of its motor apparatus. Certain ganglionic centres, 

 distinct from those of simply-reflex action, are set apart for these purposes; 

 but we can trace no essential peculiarity in their structure, which can in any 

 way indicate their possession of this extraordinary endowment. They still con- 

 sist of vesicular matter, with afferent or sensory nerve-trunks terminating in 

 them, and with efferent or motor trunks issuing from them ; and our assurance, 

 that it is through their instrumentality that the mind is affected by external 

 impressions, rests upon the fact, that if an interruption exist in the transmission 

 of the influence to which those changes give rise, at any point whatever between 

 the periphery and the central ganglion, those impressions are not felt; whilst, 

 conversely, any interruption along the course of the efferent fibres, prevents the 

 influence of psychical states from producing any respondent contractions in the 

 muscles. The simplest mode in which the Nervous system is subservient to 

 mental changes, is in that affection of the consciousness which is directly conse- 

 quent upon external impressions, and is designated as sensation ; and the part 

 of the apparatus in which this conversion is effected, is termed the sensorium. 

 We shall hereafter see that this " sensorium" probably consists, in Man, of a 

 number of distinct ganglionic centres, each of which is the instrument of some 

 one special kind of sensorial change. From these sensory ganglia, also, motor 

 fibres proceed to the muscles ; and through their instrumentality, movements 



