NERVOUS TISSUE 



153 



STRUCTURE OF NERVES. 



Nerves are bundles of nerve fibers passing between the central nervous 

 system and the various parts of the body; they are so widely distributed 

 that they may be found in sections of most of the organs and tissues. 

 When examined fresh, in reflected light, nerves are seen to be of two sorts, 

 formerly known as white and gray nerves, respectively. Similarly, sec r 

 tions of the brain and spinal cord are formed of white substance and gray 

 substance. The obvious distinction in color is due to the presence or 

 absence of microscopic sheaths of myelin around the individual fibers. 

 Nerves which contain a large proportion of myelinated or medullated 

 fibers are white; and those which have few are gray. All nerve fibers 

 when first formed are non-medullated, and most of the sympathetic nerves 

 remain in this condition. 



Non-medullated nerves can readily be found between the circular and 

 longitudinal layers of smooth muscle in any part of the digestive tube. 

 They are circumscribed bundles of fine fibers running through the coarser 

 connective tissue (Fig. 138). Many of them contain nerve cells, unmis- 



Fic. 138. A SYMPATHETIC NERVE FROM THE MYENTERIC PLEXUS OF A CAT. X 775. 



a., Nucleus of a supporting cell; b., nerve cell; c., non-medullated nerve fibers. Above the nerve are cir- 

 cular smooth muscle fibers in longitudinal section; below it are longitudinal fibers in cross section. 



takably characterized by large, round or oval, vesicular nuclei, having a 

 prominent nucleolus. Around the nucleus is dense protoplasm, starting 

 out in branching processes, all but the roots of which are cut away in 

 sectioning. Other cells are found, having relatively small nuclei and very 

 indefinite or wholly imperceptible protoplasmic bodies. These are support- 

 ing cells; they produce a syncytial framework in which the nerve cells 

 and their very delicate ramifications are imbedded. The framework 

 tends to form septa, subdividing the nerve into smaller bundles. 



Some non-medullated fibers, but by no means all, are closely invested 

 by sheath cells. According to Schafer, the nuclei of these cells appear to 

 be interpolated in the substance of the fiber, and it is impossible to demon- 



