CH. XLVIII.] AFFERENT TRACTS TO THE CORTEX 703 



In the cord, however, it gives off many collaterals, some of which 

 arborise around anterior horn cells of the same or the opposite side, 

 to form the basis of spinal reflex action. Others are shown crossing 

 the middle line, and they ultimately reach the thalamus by the 

 ascending tracts of the opposite side of the spinal cord and bulb. 



The arrangement of the sensory cranial nerves is very similar ; a 

 cell of the Gasserian ganglion is seen sending its peripheral axon to the 

 face region in the fifth nerve, and its central axon to arborise around 

 the cells of the sensory nucleus of the fifth nerve in the bulb. From 

 these cells the second relay carries on the impulse to the thalamus. 



The arrangement of the cochlear nerve is very similar ; the second 

 relay, via trapezium and lateral fillet, carries the impulse, however, to 

 the posterior corpus quadrigeminum instead of to the optic thalamus ; 

 a third relay, not shown in the diagram, completes the journey from 

 this mass of grey matter to the cortex. 



The connections of the cord and cerebrum to the cerebellum we 

 have previously studied (see fig. 422, p. 684), and so they are not 

 shown in the present diagrams. 



Particular attention should be paid to the following point : when 

 an afferent fibre enters the spinal cord, it divides into three main 

 sets of branches. The first set, the shortest, forms synapses with 

 the motor cells of the anterior horn ; here we have the anatomical 

 basis of spinal reflex action. The second set passes through an 

 intermediate cell-station in Clarke's column to the cerebellum, the 

 emerging fibres from which also influence the motor discharge of 

 the cortical and anterior horn cells. The third set, the longest, 

 passes through three intermediate cell-stations (the first in the 

 nucleus gracilis or cuneatus, the second in the optic thalamus, the 

 third in the association units in the cortex), and ultimately reaches 

 the pyramidal nerve-cells of the cerebral cortex, the efferent fibres 

 (pyramidal fibres) of which pass to the motor cells of the anterior 

 cornu and influence their discharge. The motor nerve-cells of the 

 anterior horn may thus be influenced by the afferent impulses 

 via three paths or nervous circles. In health, all these nervous 

 circles are in action to produce coordinated muscular impulses. In 

 locomotor ataxy, which is a degeneration of the cells of the ganglia 

 on the posterior roots and their branches, all these nervous circles 

 are deranged, and the result is loss of reflex action, and incoordina- 

 tion of muscular movements. 



