118 



THE INDIVIDUAL ORGANISM 



within the spinal cord or brain. In these organs we find a large variety of 

 "levels" at which synapses may take place. We have already seen that 

 a number (really a rather large number) of reflex arcs can occur within a 

 "spinal reflex" frog. Synapses may connect a receptor and an effector 

 through the dorsal and ventral roots of the same nerve, or they may con- 

 nect the left dorsal root of a given nerve with the right ventral root of its 

 mate, thus involving the crossing of the spinal cord. Other synapses may 

 unite the afferent nerve of one dorsal root with one or more efferent neu- 

 rons in spinal nerves above or below the one that received the stimulus. 

 The spinal cord thus provides for a great number and variety of neural 



<S* 



adjustor neuron 



^ "tiz\2^i 



voluntary muscle 

 motor neuron 



sensory receptor 



SPINAL CORD 



Fig. 7.7. The mechanism of first-level response, (a) Axon; (d) dendrite. (Modified from. 

 Gates, Elementary Psychology, by permission The Macmillan Company.) 



connections, and some of the white matter of the cord is formed by 

 fibers of neurons that connect the various levels of the cord with one 

 another. 



Most of the fibers of the white matter of the cord, however, connect 

 with the brain and here provide for still further synapses. 



The autonomic nervous system. There is another division of the 

 nervous system that we have as yet scarcely mentioned. As was stated 

 above, the ramus communicans, leading out from a spinal nerve, contains 

 the nerve fibers that supply the visceral organs, including smooth muscle,- 

 blood vessels, and glandular tissue. The fibers of this ramus do not go 

 directly to the visceral organs, but make synapses in the autonomic 

 ganglia, especially those of the autonomic trunks. The latter are of a pair of 

 large nerve tracts that lie along the dorsal wall of the body cavity, parallel 

 to and on either side of the aorta. There are 24 ganglia along the course 



