AND PHYSIOLOGY, 



815T 



FiG. 302. 



Tubular Nerve Fibers. 



nerve fibers. These fibers have the appearance of a double tube, 

 or a small tube within a larger one, and sometimes exhibit small 

 nucleated cells within the two. 



564. A closer examination shows an inner or grayish por- 

 tion which is called the axis cylinder, and a white substance 

 around this called the medullary matter, or substance of 

 Schwan, and outside of the whole a membranous tube. (Fig. 

 302.) They are often called Fine Nerve Fibers. The sym- 

 pathetic system, on the other hand, seems to be made u*p of 

 tubes without this double structure, and when several of 

 them are joined in a bundle, they present a grayish appear- 

 ance. They are also of a much smaller size, varying mostly 

 from T5F0 o^h to sVooth of an inch. 



565. Vesicular Structure. — The vesicular substance is 

 composed of cells or vesicles, which present very curious 

 forms, being somewhat stellate or caudate. The central por- 

 tion is globular, consisting of a nucleated cell, which sends 

 off processes in different directions, as seen in the annexed 

 Figure 303. Their diameter is exceedingly variable, meas- 



What is the structure of the sympathetic system ? 564. What still more minute struc- 

 ture can be detected by the microscope ? 565. Describe the vesicular structure. Giva 

 the diameter of the cells. 



