182 Basic Structure of Vertebrates 



autonomic system. Having entered the autonomic ganglion, whether 

 lateral or collateral, the afferent fibers merely pass through it without 

 making connection to any cell in the ganglion. Emerging from the gan- 

 glion, the afferent fiber (bound together with other fibers, chiefly ef- 

 ferent, all together constituting a visceral nerve) continues, without 

 interruption, to its visceral destination. Therefore, in its mode of dis- 

 tribution it does not differ from a somatic sensory fiber. In each case, 

 one continuous fiber derived from a cell-body in the dorsal spinal gan- 

 glion passes directly to its peripheral receptive end, whether that be in 

 somatic or in visceral territory, and regardless of the distance to be 

 traversed. (Some exceptional afferent fibers having the cell-body in an 

 autonomic ganglion have been described.) The visceral afferent fiber 

 usually has no specialized receptor other than its own terminal fibrils. 

 It is the mode of distribution of the efferent, but not the afferent, 

 fibers which distinguishes the visceral system from the somatic system. 

 A somatic efferent fiber, derived from a cell-body in the gray 

 matter of the spinal cord, passes out in the dorsal or ventral trunk of 

 the spinal nerve and continues uninterruptedly to its termination in 

 some striated muscle (Fig. 152). This direct relation necessitates 

 some incredibly long fibers, as, for example, in the case of the nerve 

 which supplies the most distal muscles in the foreleg of a giraffe. In 

 contrast to the direct relation of the somatic fiber to its effector, that of 

 the visceral efferent fiber is indirect. There are no efferent "through 

 trains" from central nervous stations to visceral destinations. Any 

 nervous impulse that makes the trip travels over two neurons. An 

 autonomic ganglion is a way station where the transfer is made at the 

 synaptic junction of the two neurons. The synapse may be in any gan- 

 glion — lateral, collateral, or terminal. The central or proximal neuron, 

 making the connection between the central organ and a ganglion, is 

 conveniently called the "connector" (or preganglionic) neuron. The 

 distal or peripheral neuron, being the one that finally delivers the im- 

 pulse to the effector, is called the "exciter" (or postganglionic) 

 neuron (Fig. 152). If the synapse is in a lateral ganglion, the connector 

 is short and the exciter may be very long. But, if the synapse is in a 

 terminal ganglion, the connector is long and the exciter may be very 

 short — e.g., connector-fibers in the intestinal trunk of the vagus nerve 

 arise from cell -bodies in the medulla and pass as far back as the hind 

 end of the "small" intestine, where they make synapses with neurons 

 of the myenteric plexus. Having entered a lateral ganglion, a connector, 

 or a branch of it, may turn and extend forward or backward along the 

 cord for a distance of several segments. Meanwhile it may give off 

 several branches, each of which ultimately makes synaptic connection 

 with an exciter. Therefore the total number of exciters greatly exceeds 



