BY DR. KLEIN. 81 



abruptly terminates, or may end in a terminal thickened bor- 

 der. In other instances, more particularly in the striated 

 muscles, the sheath very often stops suddenly without any 

 previous attenuation. 



Neurilemma. In order to make out satisfactorily the 

 relation of the nerve fibres in a nerve trunk, sections must be 

 prepared, either of nerves hardened in alcohol, or in diluted 

 chromic acid, and must then be, stained with carmine; or tis- 

 sues known to be richly supplied with nerves must be em- 

 ployed, e.g., tongue, (Esophagus, trachea, urinary bladder, etc. 

 In cross sections of nerves, the nerve fibres are seen to be in- 

 closed in a well-defined connective-tissue sheath (neurilemma), 

 of thickness more or less proportional to that of the nerve 

 itself. Between the fibres of the neurilemma, cellular struc- 

 tures are met with. In many nerve trunks, septa stretch in- 

 wards from the sheath, by which the nerve fibres are divided 

 into a greater or less number of bundles. In such preparations 

 the cross sections of each nerve fibre exhibit an external ring 

 with double contour the cut edge of the medullary sheath 

 inclosing a body of circular outline which does not fill up the 

 whole of the space, and is readily stained by carmine. In a 

 longitudinal section of a nerve we observe, within the connec- 

 tive-tissue sheath, the double contoured fibres, running parallel 

 with each other, but following a more or'less wavy course, and 

 showing the nuclei of their Schwann's sheaths. In newly-born 

 children the number of nuclei is much greater than in adults. 

 The spinal nerves, which in the frog find their way to the skin 

 from the spinal cord through the dorsal lymph sac, possess an 

 extraordinarily thick neurilemma; this is covered by a layer 

 of endothelium, which can be demonstrated by staining with 

 nitrate of silver. In the neurilemma of many microscopical 

 nerves, fine capillary vessels can oiien be made out. For the 

 tracing out of medullated fibres, the use of osmic acid is of 

 great value; for the medullary sheath is, in consequence of 

 the fatty matter it contains, stained dark by this reagent. 



Schwann's Sheath. With the exception of the optic 

 and auditory nerves, the fibres of all peripheral nerves possess 

 a Schwann's sheath. The nuclei which the sheath contains 

 are seen, when examined in the fresh state in indifferent fluids, 

 to be pale, and more or less distinctly granular. When acted 

 on by acids or hardening reagents, they shrink. In freshly 

 prepared teased preparations (e.g., of the sciatic nerve of the 

 froo;), the Schwann's sheath of the wide medullated fibres ran 

 be "recognized with great difficulty. In general, only the 

 nuclei can be made out. The sheath itself can be more easily 

 seen in the narrow non-medullatc'l fibres. In the nerves of 

 the tail of the tadpole, and of the membrana nictitans of the 

 frog, in those of the mesentery of the frog and of mammalia 

 6 



