CKLL THEORY OF NBEVB STBtlCTUEB: NEURA 405 



with fibers from many different parts of the central sys- 

 tem. The advantage of such a combination is seen in the 

 manner in which a single nerve is able to coordinate very 

 complex movements, the messages from the different cen- 

 ters of control being concentrated, as it were, by this 

 arrangement. 



One of the largest plexuses is located just above the pit 

 of the stomach, and is called the solar plexus because smaller 

 plexuses radiate from it like the rays of the sun. A blow 

 on this area paralyses many regions, and 

 may even produce death. 



Cell theory of nerve structure; neura. — 

 The presence of nuclei under the neuri- 

 lemma at once suggests the idea that 

 nerve fibers bear some relation to cells. 

 Examination of the gi'ay matter of the 

 cord or brain in which these fibers orig- 

 inate gives us further information on 

 this point, and to the relations thus 

 established is due the present theory of 

 nerve structure and action. 



Preparations of gray matter show it 

 to be rich in nucleated cells of irregular 

 outline. (See Fig. 192.) From the angles 

 of this outline project fine branches of 

 protoplasm, which may subdivide again 

 and again to form treelike gi-owths, or 

 which may continue some distance with- 

 out branching, in the form of a c}'-linder. 

 The branching outgrowths are called 

 dendrites, while the cylinders are called axis cylinders, or 

 axones. 



Fig. 192 — Nerve cells 

 from the cortex of the 

 cerebral hemispheres; 

 jix axis cylinder. 



