88 AN INTRODUCTION TO ZOOLOGY 



contradistinction to the second group of fibres, as a non-medullated 

 fibre. 



If the fibre is one that leaves the central nervous system it is 

 provided with yet another protective and insulating coat. Between 

 the primitive sheath and the axis cylinder is developed a compara- 

 tively thick layer, the medullary sheath, composed of a substance, 

 myelin, closely allied to fat. This substance is highly refractive in 

 the living condition, and consequently these nerves have a character- 

 istic double contour when seen under the microscope, but when 

 dead become white and opaque, and hence such nerve fibres are termed 

 white or medullated fibres. The neurilemma is present as before, 

 and its nuclei immediately beneath it. The medullary sheath is 

 not continuous throughout the length of the fibre, but divided into 

 a series of fairly long segments. In the short gaps between these 

 segments the primitive sheath, slightly thickened, comes into contact 

 with the axis cylinder. The ring-like constrictions formed in this 

 way are termed the nodes of Ranvier, and the portion from one node 

 to the next is an internodal segment. In the living condition the 

 medullary sheath is apparently homogeneous and continuous in the 

 internodal segment, but when dead it appears to break up into a 



series of overlapping portions, 

 the segments of Schmidt, which 

 are separated from one another 

 by clear narrow fissures running 

 obliquely to the long axis of the 

 axis cylinder. In the higher 

 vertebrates each internodal seg- 

 ment has only one elongated oval 

 nucleus lying in the neurilemma, 



E. ^?Sl|liil^ but in fish a number of nuclei 



are scattered in each internode. 



It has already been mentioned 



FIG. ^o. Transverse section of a .-> , -, ,, -, n -, -. 



nerve, that both brain and spinal cord 



are composed of grey and white 



A., artery ; E., epmeunum ; F., fasciculus ; * _,. J 



p., perineurium ; v., vein. matter. Ihe grey matter is 



mainly composed of the cell 



bodies with non-medullated axons and dendrons, while the white 

 matter is almost entirely made up of white fibres. 



If a nerve such as we find in any part of the body, e.g. the 

 sciatic nerve, be cut in transverse section it will be seen that it is 

 composed of an enormous number of fibres grouped together and 

 surrounded by a tough connective tissue envelope, the epineurium. 

 Within this the fibres are seen to be arranged in definite bundles of 

 various sizes, the fasciculi, each of these has its own sheath, the 



