NERVES. 



IOI 



NERVES are bundles of nerve fibers enveloped in connective tissue 

 sheaths. According to the nature of their constituents they are classed 

 as medullated and non-mednllated, a distinction which the student should 

 remember to record. 



The spinal and cerebral nerves consist mostly of medullated fibers 

 of varying diameter (2-20 ( ), scattered among which are a few that are 

 non-medullated. Medullated nerves are white in reflected light. They 

 are surrounded by loose connective tissue [the epineurium] which contains 

 lymphatic and blood vessels, and small nerves, and has many elastic 

 fibers. It extends around the entire nerve and between the several well 

 defined bundles of which a large nerve consists (Fig. 117). Each of these 

 bundles is covered by a dense lamellar layer of flattened connective tissue, 



Endo- 

 neurium. 



Fiber sheath. 



FIG. 118. MEDULLATED NERVE. PART OP A CROSS SECTION' OP THE HUMAN 

 MEDIAN NERVE. X 220. 



called the perineurium (Fig. 118). The cells in the perineural layers are 

 in contact with one another along their borders so that on surface view 

 they resemble a mesothelium. The perineurium sends septa into the 

 nerve bundle and becomes continuous with the connective tissue which, 

 outside of the neurolemma, surrounds each individual nerve fiber [Henle's 

 sheath]. The inner extensions of the perineurium may be called internal 

 perineurium (or endoneurium). The perineurium contains capillaries, 

 generally parallel with the nerve fibers, and tissue spaces, but no lymphatic 

 vessels. The outer sheaths of the nerves are continuous with the dura 

 mater of the cord and brain. 



The large sympathetic nerves vary in color. The splanchnic nerves 

 contain many medullated fibers and are whiter than the nerves of the 

 plexuses. Medullated fibers in the latter are few and very slender. Non- 

 medullated nervous tissue is gray. A part of the medullated fibers of the 

 sympathetic nerves come directly from the spinal nerves, and a part are 

 medullated processes of the sympathetic ganglion cells. Small non- 



