652 STRUCTURE OF THE CEREBRUM. [CH. XLVI. 



of the spinal cord are more fully shown in figure 453. 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 (P), 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. 481 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 colla- 

 terals 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 arbori- 

 sation 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 sympathetic 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 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 co-ordinated 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 incoordination of muscular movements. 



The Convolutions of the Cerebrum. 



The surface of the brain is marked by a great number of de- 

 pressions which are called fissures or sulci, and it is this folding 



