90 HUMAN PHYSIOLOGY 



the posterior columns, and, crossing the middle line, run up 

 to the base of the great brain, where they end in synapses 

 round other cells in the thalamus opticus. 



(6) Upper Neurons. From these cells, processes pass up 

 to the surface of the great brain, to end in synapses with the 

 cells situated there. 



B. Outgoing (D). These start in the cells of the cortex 

 cerebri, and pass down to the upper part of the spinal cord, 

 where most of them cross and run down the lateral column 

 of the spinal cord, giving off collaterals which end in 

 synapses round the cells in the anterior horn of grey matter, 

 from which the spinal outgoing neurons pass to the muscles, 

 &c. Those which do not cross run down the anterior column 

 of the cord for some distance, and end by crossing and 

 becoming associated with the cells in the anterior horn. 



3. Cerebellar Arc A. Ingoing (E). Some of the collaterals 

 of the spinal ingoing neuron end in synapses round a mass 

 of nerve cells at the side of the grey matter of the spinal 

 cord the cells of Lockhart Clarke. From these cells, fibres 

 extend up at the side of the cord to the lesser brain or 

 cerebellum, to form, directly or indirectly, synapses round 

 the cells in this organ. 



B. Outgoing (F). From cells near the surface of the cere- 

 bellum and in the roof nucleus (a) axons extend to the 

 medulla oblongata, where they end in synapses round a mass 

 of cells the nucleus of Deiters. From these cells, fibres 

 extend down the lateral columns of the spinal cord, and 

 give off collaterals to the cells of the anterior horn of grey 

 matter. (6) Other neurons pass to the cerebrum. 



The nervous system may thus be considered as built 

 up of these three sets of arcs. 



1st. The Spinal arcs, consisting of the peripheral ingoing 

 neurons and the peripheral outgoing neurons. These arcs 

 are not only at the level of the cord at which the ingoing 

 neuron enters, but at various levels above and below this 

 point. 



2nd. The Cerebral arcs, consisting of the peripheral, the 

 intermediate and the upper ingoing neurons, and the central 

 outgoing neurons, and the peripheral outgoing neurons. 



3rd. The Cerebellar arcs, consisting of peripheral ingoing 



