SUMMARY 421 



strained in trying to follow it. We ought not to read 

 while lying down; the position is awkward, and trying 

 to the eyes. 



419. Summary. The body is one organism. The nervous sys- 

 tem regulates the body by providing communication between its parts. 

 The nervous system consists of the central and the sympathetic 

 systems. 



Neurons are nerve cells. They consist of cell bodies, and of 

 nerve fibers, or axons. Dendrites are short extensions of cell 

 bodies. 



A nerve is a bundle of nerve fibers. A nerve message is called a 

 stimulus, or nerve impulse. Nerve fibers are of 3 kinds: afferent 

 (sensory), efferent (motor), and associating. 



Groups of neurons are called ganglia. 



The brain consists of forebrain, midbrain, and hindbrain. 



The cerebrum is the forebrain. It communicates with the body 

 through the midbrain, hindbrain, and spinal cord, but it has also direct 

 connection, by means of cranial nerves, with parts of the head, 

 neck, and trunk. 



The cerebrum is the organ of the mind. 



The hindbrain consists of the cerebellum, pons, and bulb. 



The spinal cord is enclosed in the spinal column. Its out- 

 side consists of white nerve fibers; its inside of gray cell bodies. It 

 has ma'ny pairs of nerves; each nerve has afferent and efferent 

 fibers. 



The sympathetic system consists of ganglia and nerve fibers that 

 control the internal organs and the skin. The sympathetic system 

 relieves the central system of much labor, but it cannot, of itself, send 

 back efferent impulses to the organs. 



Nerve connections are commonly made by the interlacing of the 

 "end brush" of a nerve fiber with the dendrites of a neighboring cell 

 body. 



The conduction pathway taken by a nerve impulse is the one that 

 offers the least resistance. 



