642 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



ganglionic centres has reference, not to the multiplication of similar parts which 

 are to be alike supplied with nervous power, but to the exercise of adversity of 

 functions, through the instrumentality of different structures ; thus, in the 

 higher Articulated and Molluscous tribes, we find ganglionic centres specially 

 set apart for the actions of deglutition and respiration, as well as for those of 

 locomotion ; but the modus operandi is still the same, these actions being all 

 " excito-motor," that is, being performed through the "reflex" agency of their 

 several ganglionic centres, without the necessary intervention of consciousness. 

 These centres are connected with each other commissurally, when they are re- 

 quired to act with consentaneousness j and it is frequently to be observed in the 

 most developed forms of each type, that they come into actual coalescence, their 

 functional distinctness being still indicated, however, by the distribution of their 

 nervous trunks. 



IV. In all but the very lowest Invertebrata, the nervous system includes, in 

 addition to the foregoing, certain ganglionic centres, situated in the neighbor- 

 hood of the entrance to the digestive cavity, and connected with certain organs, 

 which, from their more or less close resemblance to our own instruments of 

 special sense, we conclude to be organs of sight, smell, hearing, &c. Now as 

 we know from our own experience, that impressions made upon these organs 

 produce no further change unless we become conscious of them, and as the In- 

 vertebrata possess no distinct ganglionic centres of a higher character, it seems 

 to be a legitimate inference that these "sensorial" ganglia are the instruments 

 by which the animals furnished with them are rendered cognizant of such im- 

 pressions, and through which the sensations thus called into existence serve to 

 prompt and direct their movements. What is commonly designated as the 

 "brain" of Invertebrata (more properly their "cephalic ganglia") cannot be 

 shown to consist of anything else than an assemblage of sensorial centres ; and 

 its actions appear to be entirely of a "reflex" character, such of the movements 

 of these animals as are not excito-motor being performed (there is strong reason 

 to believe) in respondence to sensations excited by internal or external impres- 

 sions. Such movements, therefore, may be designated as sensor i-motor, or con- 

 sensual. Like the preceding, they must be accounted purely "automatic," since 

 neither emotion, reason, nor will has any participation in them ; and the propor- 

 tion which they bear to the actions of the excito-motor kind seems to correspond 

 pretty closely with the relative development of the cephalic ganglia and of the 

 rest of the nervous system, is very obvious when the larva and imago states 

 of Insects are compared. However disjoined the various excito-motor centres 

 may be amongst each other, we uniformly find them connected with the cephalic 

 ganglia by commissural tracts ; and this anatomical fact, with many phenomena 

 which observation and experiment upon their actions have brought to light, 

 makes it apparent, that besides the reflex actions which are performed through 

 their own direct instrumentality, the sensory ganglia have a participation in 

 those performed through other ganglionic centres. Thus it seems probable that 

 a stimulus transmitted downwards from the sensory ganglia, to one of the gan- 

 glia of the trunk of a Centipede, excites the efferent nerves of that ganglion to 

 call into contraction the muscles supplied by them, just as the excitor influence 

 arriving at that ganglion through its own afferent nerve would do. 



675. The whole Nervous System of Invertebrated animals, then, may be 

 regarded as ministering entirely to automatic action ; and its highest develop- 

 ment, as in the class of Insects, is coincident with the highest manifestations of 

 the " instinctive" powers, which, when carefully examined, are found to consist 

 entirely in movements of the excito-motor and sensori-motor kinds. When we 

 attentively consider the habits of these animals, we find that their actions, 

 though evidently adapted to the attainment of certain ends, are very far from 

 evincing a designed adaptation on the part of the beings that perform them, 



