146 



PRINCIPLES OF ANIMAL BIOLOGY 



whose afferent fiber enters the cord by one spinal nerve, while the efferent 

 fiber leaves it by way of the nerve next below. The lower level is 

 reached by an association neuron between the receptor and effector. 

 This same receptor is represented as connected also with an effector 

 neuron on the opposite side of the spinal cord. A second association 

 fiber establishes this connection. Some association neurons take the arc 

 through the brain, across a number of cells, and back down the spinal 

 cord again. Many arcs much more complicated than these exist. In 

 all cases the first neuron in the chain is an afferent, the last one an 

 efferent. All the contacts between any of the neurons are synapses, 

 axon touching dendrite. 



The response to a stimulus carried over a reflex ai'c is called a reflex 

 action. Many of these actions are inherited. The vital organs in the 

 chest and abdomen are controlled by innate reflexes, as are also the con- 



FiG. 121. — Chain of efferent neurons in human- autonomic system, in chest region, n, the 

 neurons; g, gangUon. Dotted lines represent neurons of ordinary spinal reflex arc. 



traction and dilation of blood vessels and the action of sweat glands. 

 Other reflexes are learned — "conditioned" is the usual descriptive term 

 applied to them. Habitual movements of all sorts are conditioned 

 reflexes. 



Functions of Autonomic System. — The reflexes for which the auto- 

 nomic nervous system is responsible are of such vital importance and 

 are related to one another in so remarkable a manner as to call for 

 separate description. Attention will be directed only to the efferent 

 fibers of the reflex arcs, because it is their control of the vital organs 

 with which we will be concerned. The system in man is the one used 

 for illustration. 



The neurons of the autonomic system lack a myelin sheath. Between 

 the central system and the organ innervated there are always at least 

 two, and often only two, neurons. In the chest region the bod}^ of the 

 first neuron of such a chain is in the lateral part of the Il-shaped gray 

 matter of the cord (Fig. 121), and its axon passes out through the ventral 

 root of one of the spinal nerves. It leaves that root, however, close to 

 the cord and enters a special ganglion. Here the first neuron terminates, 

 its axon synapsing with the dendrite of the second neuron of the chain. 



