154 



HISTOLOGY 



strata a distinct sheath (Fig. 139). Similarly Bardeen has stated that it 

 is "mainly a matter of judgment to decide whether the fibrils are sur- 

 rounded by or imbedded within the sheath cells." They correspond with 

 the neurolemma cells of medullated nerves. 



Medullated Nerves. The larger sympathetic nerves contain a consider- 

 able number of medullated fibers, and the splanchnic nerves are described 

 as white. In the trunks of the spinal nerves, however, the medullated 

 fibers attain their maximum development. Examined with low magnifi- 



FIG. 139. NON-MEDULLATED NERVE FIBERS. X 4o. (After Schafer.) 



cation, such a nerve is seen to consist of round cords imbedded in loose 

 connective tissue (Fig. 140). This loose tissue, which surrounds the 

 entire nerve and its several cords, is the epineurium; its connective tissue 

 bundles are chiefly longitudinal, and are associated with abundant elastic 

 tissue and frequent fat cells; it contains the blood vessels which supply 

 the nerve. Each cord is surrounded by a dense lamellar layer of connect- 

 ive tissue, which contains flattened cells in contact with one another so 

 that they form more or less continuous membranes. This layer is the 



Fat cells. 



Artery. 



Bundles of nerve fibers 



Epineurium. 



Perineurium. 



Endoneurium. 



FIG. 140. MKDULLATBD NERVE. PART OF A CROSS SECTION OF THE HUMAN MEDIAN NERVE. X ao. 



perineurium. It is continuous with the outer membranes covering the 

 cord, and contains cleft-like spaces which are said to communicate with 

 the subdural and subarachnoid spaces, but which do not connect with 

 lymphatic vessels in the epineurium. Prolongations of the perineurium 

 extend as septa into the larger nerve bundles and constitute the endoneu- 

 rium, which may penetrate between the individual nerve fibers, forming 

 the so-called "sheaths of Henle." Their nuclei are always outside the 

 neurolemma. 



