320 THE SENSORY TRACT. 



Fig. 160. 



D 



The sensory tract. 



avoidable that the chain of ganglia now under consideration must con- 

 stitute a sensorium, the centripetal fibres communicating their impression 

 and motion ensuing, the impressions being attended with consciousness. 

 This view is moreover substantiated by observations made after excision 

 of the cerebrum, a certain degree of consciousness remaining, not unlike 

 that exhibited by a man who is half asleep. This condition of things is 

 naturally presented in the amphioxus. 



But after the cerebral hemispheres are added, an impression received 

 Effect of the ad u P (:)n ^ ne thalamus, whether it has come in through the sen- 

 dition of the sory ganglia, or any other sensory part of the cranio-spinal 

 axis, is transmitted to the convolutions along the radiating 

 fibres. From the convolutions, the influence which is to produce mo- 

 tion descends along the converging fibres to the striatum, thence along 

 the inferior layers of the crus, through the mesocephalon to the anterior 

 pyramids, and by their decussation to the opposite side of the cord. 



Such is the view which Dr. Carpenter presents of the functions of the 

 sensory ganglia and spinal axis ; or, employing the terms we have pre- 

 viously defined, the cord alone is a longitudinal series of automatic arcs ; 

 on the addition of the thalamus and striatum, it becomes a compound 

 registering arc, the cerebral hemispheres finally annexed to it constitut- 

 ing an influential arc. 



In a simple arc, an impression is at once converted into motion, and 

 leaves behind it no traces ; its expenditure is instantaneous and complete. 

 In a registering arc, a part of the impression is stored up or remains 



