CONSTRUCTION OF THE NERVES. 



325 



This common sheath has received the name of epineurium (Key and Retzius) ; it 

 was formerly termed the "cellular sheath." 



The special sheath of a funiculus, termed the perineurium l (fig. 379, per, 

 380, P.), is also formed of connective tissue, but is far more distinctly of a lamellar 

 nature, and indeed may be stripped off in the form of a tube from the little bundle 

 of nerve-fibres of which the funiculus consists. The perineurium is not formed of 

 a single lamella but of several, which are separated from one another by inter- 



Fig. 380. PART OF A SECTION OP ONE OF THE FUNICULI OF THE SCIATIC NERVE OF MAN. 

 MAGNIFIED (after Key and Retzius). 



P, perineurium, consisting of a number of closely arranged lamellae. En, processes from the 

 perineurium, passing into the interior of the funiculus, and becoming continuous with the endoneurium, 

 or delicate connective tissue between the nerve-fibres. The connective tissue fibrils of the endoneurium 

 are seen cut across as fine points, often appearing to ensheath the nerve-fibres with a circle of minute 

 dots (fibril-sheath of Key and Retzius). Numerous nuclei of connective tissue cells are imbedded in the 

 endoueurium ; v, section of a blood-vessel. 



lamellar clefts moistened with lymph. The separation is not everywhere complete, 

 for here and there bundles pass across, connecting the several lamellae. Moreover, 

 the outermost lamella is joined by connective tissue bundles and laminae of the 



Fig. 381. A PORTION OF A SMALL NERVE-TRUNK FROM THE THORAX 



OF A MOUSE, TREATED WITH NITRATE OF SILVER (Ranvier). 



\ MAGNIFIED. 



Oross markings are seen at the nodes, and the layer of flattened 

 epithelioid cells which covers the surface is brought into view by 

 the silver deposit. 



epineurium, and the innermost gives off flattened pro- 

 longations (fig. 379, end.\ to form imperfect septa 

 between the groups of nerve-fibres within the funi- 

 culus. 



Although the lamellae of the perineurium are very 

 thin, each is formed of at least three strata. Thus 

 the main substance of the lamella is composed of a 

 connective tissue, in which both white fibres and 

 elastic elements are found, the white fibres having for 

 the most part a transverse disposition. The elastic 

 elements lie in greater abundance nearer the surfaces 



of the lamella, and often occur in the form of patches or incomplete membranes of 

 elastic substance (fig. 282, p. 240), as well as in the form of fine network of 

 fibres. On both its surfaces each lamella is entirely covered with a layer of delicate 

 flattened endothelial cells, which thus serve also to bound the clefts between the 

 lamellje. The outlines of the cells are brought into view by the silver treatment 

 (fig. 381). 



The funiculi of a nerve, although not all of one size, are all sufficiently large to 



Formerly known as the neurilemma. 



