NERVE 



113 



while from the thalamus fresh fibres pass on to the 

 cortex to end in the area situated round the central 

 fissure. 



Each of these — tectum, thalamus, and cortex — is a 

 shunting station, from which impulses are sent down the 

 cord to act upon the spinal reflex arcs. 



1. Spino-tectal Synapses. — In the tectum these incoming 

 impulses from the body are associated with incoming 

 impulses from the eye and ear (fig. 50, C.Q). 



If the brain-stem be cut above the tectum, as it is in the 



Fig. 50. — To show the endings in the thalamus of the ingoing fibres from 

 all the receptor mechanisms. S., ingoing fibres from the body 

 generally; F7//., ingoing fibres from the cochlea ; T., ingoing fibres 

 of the fifth cranial nerve ; //., ingoing fibres of the optic nerve ; C.Q., 

 tectum. (Elliot Smith.) 



process of decerebration, the tectal synapses and the 

 cerebellar connections are left intact. 



The action of these, uncontrolled by the upper cerebral 

 synapses, is to set up an extensor tone of the muscles 

 known as decerebration rigidity. In man the disconnection 

 of the cerebral cortex from the cord by the interrup- 

 tion of the pyramidal tracts brings about this condi- 

 tion with a characteristic increase in the spinal reflexes 

 (p. 82). 



2. Spino-thalamic Synapses. — It was from the thalamus 



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