774 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



and that, like other such agencies, the mode of its manifestation is affected by 

 the nature of the material substratum through which its influence is exerted. 

 Thus, the Physiologist knows full well that the immediate operation of the Will 

 is not upon the Muscles, but upon the Brain, wherein it excites that active state 

 of Nervous matter, which he designates as the operation of Nerve-force ; and 

 that the propagation of this force along the Nerve-trunks is the determining 

 cause of the Muscular contraction, which is the immediate source of the motor 

 power. He knows, too, that this dynamical metamorphosis is effected (like 

 every other analogous change) by the intermediation of a peculiar material 

 substratum, which itself undergoes a change of condition ; the components both 

 of the Nervous and Muscular substances ceasing to exist under their previous 

 forms, and entering into new combinations. Thus, then, we have evidence, in 

 what we know of the physiological conditions under which Mind produces Motion, 

 that certain forms of Vital Forces constitute the connecting link between the 

 two ; and it is difficult to see that the dynamical agency which we term Will is 

 more removed from Nerve-force, on the one hand, than Nerve-force is removed 

 from Motor force on the other. Each, in giving origin to the next, is itself ex- 

 pended, or ceases to exist as such; and each bears, in its own intensity, a precise 

 relation to that of its antecedent and its consequent. But we have not only evi- 

 dence of the excitement of Nerve-force by Mental agency; the converse is equally 

 true, Mental activity being excited by Nerve-force. For this is the case in every 

 act in which our Consciousness is excited through the instrumentality of the 

 Sensorium, whether its condition be affected by impressions made upon Organs 

 of Sense, or by changes in the state of the Cerebrum itself; a certain active condi- 

 tion of the nervous matter of the Sensorium, being (we have every reason to be- 

 lieve) the immediate antecedent of all consciousness, whether sensational or 

 ideational. And thus we are led to perceive that, as the power of the Will can 

 develop Nervous activity, and as Nerve-force can develop Mental activity, there 

 must be a Correlation between these two modes of dynamical agency, which is 

 not less intimate and complete than that which exists between Nerve-force on 

 the one hand, and Electricity or Heat on the other ( 365). This idea of Cor- 

 relation of Forces will be found completely to harmonize with those phenomena 

 already referred to, which unmistakably indicate the influence of physical con- 

 ditions in the determination of mental states ( 803) ; whilst, on the other 

 hand, it explains that relation between Emotional excitement and bodily change, 

 which is manifested in the subsidence of the former when it has expended itself 

 in the production of the latter ( 797). And further, it will be found no less 

 applicable to the explanation of all that automatic action of the Mind, which 

 consists in the succession of ideas, according to certain " laws of thought/' with- 

 out the exercise of any control or direction on the part of the individual to whose 

 consciousness they present themselves, and which manifests itself in the action of 

 those ideas upon the centres of movement. For this succession must be regarded 

 as the exponent of a series of changes taking place in the Cerebrum itself, in 

 respondence to impressions made upon it; whilst the motions which proceed from 

 these must be considered as being no less the results of its " reflex" operation, 

 than are the "consensual" of the reflex action of the Sensory Ganglia, and the 

 " excito-motor" of that of the Spinal Cord. 1 For all Physiological purposes, then, 



1 The application of the doctrine of "reflex action" to the Brain was fully developed 

 by Dr. Laycock of York, in a paper " On the Reflex Function of the Brain," read before 

 the Medical Section of the British Association at its meeting in York, Sept. 1844, and after- 

 wards published in the " Brit, and For. Med. Rev.," vol. xix. Not having; recognized what 

 appears to the Author the essential distinction, both in their anatomical and physiological 

 relations, between the Sensory Ganglia and the Cerebrum or Hemispheric Ganglia, Dr. 

 Laycock did not mark out the distinction between the " sewsorz-motor" or "consensual" 

 actions, which are the manifestations of the reflex power of the former, and the " ideo- 



