CH. XLVI.] MAIN TRACTS 661 



The motor nerve-fibre passes from the anterior cornual cell to muscular fibres, where 

 it ends in the terminal arborisations called end-plates. 



Coming now to the sensory fibres, a cell of one of the spinal ganglia is shown. 

 Its axis-cylinder process bifurcates, and one branch passes to the periphery, ending 

 in arborisations in skin and tendon. The other (central) branch bifurcates on entering 

 the cord, and its divisions pass upwards and downwards, the latter for a short 

 distance only ; the terminations of this descending branch and of collaterals of the 

 ascending branch round the cells of the spinal cord are more fully shown in fig. 448. 

 The main ascending branch arborises around a cell of the nucleus gracilis (N.G.) or 

 nucleus cuneatus in the posterior ^columns of the bulb ; the axis-cylinder process of 

 this cell passes over to the other side as an internal arcuate fibre (I.A.), and becomes 

 longitudinal as one of the fibres of the mesial fillet (r), which terminates round a cell 

 of the optic thalamus (O.T.), from which a new axis-cylinder process passes to form 

 an arborisation around the dendrons of one of the cerebral cells (Cajal's nerve-unit 

 of association A.C.N.) in the surface layer of the cortical grey matter (shown on a 

 larger scale in fig. 479 F); the axis-cylinder process of A.C.N. arborises round the 

 dendrons of the pyramidal cell from which we started. 



In this way one gets a complete physiological circle of nerve-units ; the segments 

 of the circle are, however, anatomically distinct, and the impulses travel through 

 contiguous, not through continuous, structures. The simple arrows indicate the 

 direction of the impulses in the efferent projection system ; the feathered arrows in 

 the afferent projection system. 



Next we come to the connections of the cerebellum. One of the collaterals of 

 the sensory nerve-fibre arborises round a cell of Clarke's column, from which a fibre 

 of the direct cerebellar tract passes to end in an arborisation around a cell in the 

 vermis of the cerebellum, p is one of the cells of Purkinje, the axis-cylinder process 

 of which P. ax passes to the cerebro-spinal axis; it is depicted as passing down to 

 envelop one of the cells of the anterior horn ; but this has never been satisfactorily 

 demonstrated ; so a dotted line has been used to indicate this uncertainty. No 

 doubt also some of its collaterals pass up to the cerebrum. 



The origin and destination of the tract of Gowers are not shown in the diagram ; 

 the fibres of communication from the cerebral to the opposite cerebellar hemisphere, 

 which pass through the superior cerebellar peduncle, are also omitted. The 

 sympathetic system, with its numerous cell stations in the sympathetic ganglia, we 

 have studied in connection with the blood-vessels and viscera to which the sympa- 

 thetic fibres are distributed (see especially pp. 299-303). 



G.M. is the grey matter which is continuous from spinal cord to the optic 

 thalamus, and through this certain afferent impulses, such as those of pain, travel 

 upwards. 



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 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 associa- 

 tion 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 



