GENERAL SUMMARY. 379 



nature, and are destined to enable them to receive communications, and to act 

 on the muscular system, through the mediation of the latter, than that they 

 are actually continuous with any of the fibres in the nerve-trunks connected 

 with it (see 473). 



498. According to these views, the following will be the mechanism of the 

 different classes of actions, in which the Cerebro-Spinal apparatus is directly 

 concerned. 



i. In Reflex movements, a stimulus acting through the excitor fibres upon 

 the vesicular matter of certain parts of the Spinal Cord, causes the transmis- 

 sion of a reflex impulse through the motor fibres that proceed from it ; and 

 this gives occasion to muscular contraction. With this operation, sensation 

 will be coincident, if the stimulus act upon any of the fibres that pass on to 

 the Sensory ganglia ; but this is not essential to it ; and will not be aroused 

 if the connection does not exist, or the Sensory ganglia be in a state of torpor. 



ii. In Sensation, the stimulus acts upon fibres which have their termina- 

 tion in the chain of ganglia that lies at the base of the cranial cavity in Man, 

 and is closely connected with the Medulla Oblongata. The series is collect- 

 ively termed the Sensorium ; but it is probable that each is the instrument, 

 by which the animal becomes cognizant of Sensations of a particular class, 

 the Olfactive, Optic, and Auditory ganglia, for those of Smell, Light, and 

 Hearing respectively, the Thalami Optici for those of Touch, and certain parts 

 of the Medulla Oblongata for those of Taste. 



in. In Consensual movements, the stimulus conveyed by the Sensory fibres 

 becomes the direct source of motor impulses ; which are conveyed through the 

 agency of fibres that issue from the Sensory ganglia and Corpora Striata. 

 All the movements which are neither reflex nor voluntary, seem to belong to 

 this class; which will include, therefore, the instinctive actions of the lower 

 animals, with the automatic and purely emotional movements in Man. 



iv. In the act of Perception, or the formation of ideas from Sensations, in 

 Memory, and in all the higher acts of Mind, the Cerebrum seems to be con- 

 cerned ; the vesicular matter which constitutes its active portion, receiving the 

 stimulus to its operations, through the ascending and commissural fibres that 

 connect its different parts with the Sensory Ganglia at its base. As the con- 

 ducting power of these fibres acisfrom, not towards, the Sensory ganglia, we 

 should not expect that irritation of them should produce Sensation ; and this 

 is precisely what experiment shows to be the case. 



v. In the act of Voluntary movement, which results from mental operations, 

 the vesicular matter of the Cerebrum operates, through the descending and 

 commissural fibres, upon the motor portion of the Sensory ganglia ; the 

 stimulus transmitted downwards by Volition producing the same kind of state 

 in its vesicular matter, as that which is transmitted upwards by Sensation. 

 In the same manner, the recal of past Sensations and Ideas may reproduce, in 

 the Sensory ganglia, the condition which gives occasion to the purely Emo- 

 tional movements. 



vi. The combination and harmonization of the separate acts of Voluntary 

 Muscular movement, which is the function here attributed to the Cerebellum, 

 appears to be prompted by the guiding sensations, of which the Sensory 

 ganglia are the seat; the influence of these will be propagated along the com- 

 missural fibres known as the processus a cerebello ad testes; and the motor 

 influence, resulting from the action thus excited in the vesicular matter of the 

 Cerebellum, will be propagated downwards by its connections with the various 

 columns of the Spinal Cord. 



499. The distinctness of the operations of these several centres is shown 

 in various ways: but especially by conditions of the bodily system, in which 

 one or more of them is in a state of inaction, whether temporary or permanent; 



