SECTION IV 

 THE NEURO-MUSCULAR MECHANISM 



THE study of the physiology of muscle and nerve leads to the 

 consideration of how the neuro-museular mechanism acts, so 

 that (1st) the various visceral muscles respond appropriately 

 to the conditions in which they are placed, and (2nd) so 

 that the co-relationship of the animal with its surroundings 

 may be maintained. 



We shall at present deal with the second of these, leaving 

 the former for consideration when studying the physiology of 

 the viscera. 



1. THE NEURAL ARCS 



The neuro-muscular mechanism is controlled by three chains 

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

 side and outgoing neurons on the other. 



1. Spinal or Peripheral Are A. Ingoing (fig. 40, A). 

 These neurons start in dendritic expansions at the periphery, 

 and enter the cord by the superior or dorsal roots of the spinal 

 nerves. In these roots they are connected with cells by 

 lateral branches (see p. 158). When they enter the cord they 

 either pass to the dorsal 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 synapses 

 round masses of cells the nuclei of the dorsal columns, or, 

 (c) either directly or by collaterals, they form synapses with 

 other neurons. The most important of these in the spinal arc 

 are the neurons in the ventral part from which the outgoing 

 fibres spring. The other neurons with which synapses are 

 formed send fibres either up the same side of the cord or 

 across to the opposite side. These may be considered as part 

 of the next two arcs (see p. 170). 



