098 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



arises, why Sensation should so constantly participate in these operations, if not 

 essential to them ; and the answer to this question is to be found in the fact 

 that it is only through sensation that a higher set of actions, mental and bodily, 

 is called into play, which is essential to the continued maintenance of those 

 belonging to the present category. Illustrations of this truth might be drawn 

 from any one of the functions already noticed; but the Ingestion of food will 

 supply us with one of the most apposite. We have seen that the act of Deglu- 

 tition is in itself independent of sensation ; anything that comes within the grasp 

 of the pharyngeal constrictors being conveyed downwards by their reflex con- 

 traction, just as anything which touches the arms of a Polype is entrapped by 

 them and drawn into the stomach. Now this action may be considered as 

 attended with sensation, in the ordinary condition of the higher Animal, in 

 order that it may be guided in the performance of those other movements of 

 prehension, mastication, &c., by which the food may be brought within reach 

 of the apparatus of deglutition ; and the sensations which are linked with these 

 are among the influences which prompt to those mental operations whereby food 

 is provided for the digestive apparatus to make use of. The Zoophyte is depend- 

 ent for its supplies of aliment upon what the currents in the surrounding fluid, or 

 other chances, may bring into its neighborhood; and if these should fail, it 

 starves. The encephalous Infant, again, can swallow, and even suck; but it 

 can execute no other movements adapted to obtain the supply of food continually 

 necessary for maintenance, because it has not a mind which sensations could 

 awake into activity. The sensation connected with excito-motor actions has 

 not only this important end, but it frequently contributes to enjoyment, as in 

 Suction and Ejaculatio seminis. The sensation accompanying the actions of 

 this class, moreover, frequently affords premonition of danger, or gives excite- 

 ment to supplementary actions destined to remove it, as in the case of Respiration ; 

 for where anything interferes with the due discharge of the function, the uneasy 

 sensation that ensues occasions unwonted movements, which are more or less 

 adapted to remove the impediment, in proportion as they are guided by judg- 

 ment as well as by consciousness. Again, sensation often gives warning against 

 inconvenience, as in the Excretory functions; and here it is very evident that 

 its purpose is not only (if it be at all) to excite the associated muscles necessary 

 for the excretion, but actually to make the will set up the antagonizing action 

 of the sphincters ( 434). 



726. We have now to inquire how far the independent action of the Spinal 

 Cord is concerned in the general muscular movements of Man, and especially 

 in the locomotive actions of his lower extremities. On this point it is obvious 

 that we must not be guided by the analogy of the lower animals ; since in Man 

 the locomotive and other movements are for the most part volitional and pur- 

 posive, and he has to acquire by experience that control over his muscular appa- 

 ratus which is necessary to enable him to perform them ; whilst in Invertebrata 

 generally, and in a large part of the lower Vertebrata, it is evident that the 

 movements of progression, &c., which are characteristic of each species, come 

 under the general category of automatic actions, and are provided for in the 

 original organization of its nervous centres, being performed without any edu- 

 cation, and under circumstances which render the notion of a purpose on the 

 Animal's own part quite untenable. In so far as these instinctive movements 

 require the guidance and direction of sensations, they must be referred to the 

 " consensual" group ; but clear evidence is afforded by the continuance of many 

 of them after the removal of the centres of sensation, that they are excito-motor 

 in their character, and that they require no higher centre than the ganglia 

 which correspond to the Spinal Cord of Man. 1 There can be little doubt that 



1 See"Princ. of Phys., Gen. and Comp.,"gg 769-771, Am. Ed. 



