THE TISSUES 



89 



or arcs of neurons, consisting of ingoing neurons on the one 

 side, and outgoing neurons on the other. 



1. Spinal or Peripheral Arc A. Ingoing (Fig. 40, A). 

 These neurons start in dendritic expansions at the peri- 

 phery, and enter the cord by the posterior roots of the spinal 

 nerves. In these roots they are connected with cells by lateral 

 branches (see p. 142). When they enter the cord they pass 

 to the posterior portion, and divide into (a) branches running 

 for a short distance down the cord; (6) branches run- 

 ning right up to the top of the spinal cord to end in 



I'iC-. 40. To show the three Arcs in the Central Nervous System. A, Peripheral 

 ingoing neuron giving off collaterals in the cord and terminating above in the 

 nuciM of posterior columns ; B, peripheral outgoing neurons ; C, ingoing 

 cerebra 1 neurons; D, outgoing cerebral neurons, decussating at// above the 

 cord ; E, r'ngoing cerebellar neurons ; F, outgoing cerebellar neurons. 



synapses round masses of cells the nuclei of the posterior 

 columns. 



From these, collateral branches are given off which pass 

 forward, and, for the most part, end in synapses adjacent to 

 cells placed in the front part of each side of the cord. 



B. Outgoing (Fig. 40, B). From these cells, fibres are given 

 off which pass out in the anterior roots of the spinal nerves 

 to muscles, glands, and other reacting structures. The action 

 of these neurons is controlled and modified by the two other 

 series of central neurons. 



2. Cerebral Arc A. Ingoing (Fig. 40, O) (a) Lower 

 Neurons. These are (i.) ingoing neurons of the spinal arc 

 which lead up to the top of the spinal cord and end in 

 synapses in the nuclei of the posterior column ; (ii.) inter- 

 mediate neurons. These start from the cells in the nuclei of 



