90 VETERINARY PHYSIOLOGY 



3rd. The Cepebellar arcs consist of (1) peripheral ingoing 

 neurons; (2) the cerebellar ingoing neurons; (3) the outgoing 

 cerebellar neurons, either direct to the cord or through the 

 cerebrum ; and (4) the peripheral outgoing neurons. 



2. MODE OF ACTION OF THE NEURAL ARCS 

 A. Spinal Ares in Reflex Action 



The mode of action of the spinal arcs, uncomplicated by the 

 influence of the two higher arcs which act upon them, may best 

 be studied by dividing the spinal cord high up so as to remove 

 from it their modifying action. 



One of the simplest spinal reactions may be studied in 

 a frog in which the brain has been destroyed. If the animal 

 be suspended by the head, and one of the toes gently 

 pinched, the leg is drawn up {Practical Physiology). Here 

 an impulse is transmitted up from the ends of the receiving 

 neuron in the skin to the cord, and this leads to changes 

 in outgoing neurons which pass to the muscles of the leg to 

 make them act in a definite manner to withdraw the leg 

 from the source of irritation. This is a Reflex Action, an 

 action in which, apart from any necessary implication of 

 consciousness, an external stimulus leads to a definite re- 

 action through the agency of one or more spinal arcs. 



The mechanism involved consists in : 



1. The special terminations of the neurons acted upon the 

 Receptors. 



2. The Conductors the ingoing and outgoing neurons and 

 the synapses between these. 



3. The reacting organs or muscles, which may be termed 

 the Effectors. 



It is convenient to deal first with the conductors. 



The Conductors. These are (1) the ingoing neurons; (2) 

 the synapses ; (3) the outgoing neurons. The mode of action 

 of this chain differs materially in several points from the action 

 of single neuron fibres or groups of these which make up the 

 nerve trunk. 



1. Conduction is slower, i.e. there is a longer interval, a 

 longer latent period, between the application of the stimulus 

 and the resulting action. Thus the time between the appli- 



