MEDULLARY SEGMENTS. 313 



of nerves in length with the growth of the limbs and other parts of the body is in 

 part due to interstitial elongation of the segments. 



In the middle of each internode an oval nucleus lies embedded in the medullary 



Fig. 361, MEDULLA.TED NERVE-FIBRE TREATED WITH OSMIC ACID (Key and Retzius). 



A node of Ranvier (E) and a nucleus (K) is represented. The medullary sheath 

 appears broken up into a number of segments with conical or funnel-shaped ends fitting 

 into one another. 



sheath (figs. 356, B, 359, c, 361, K); these nuclei will be described with 

 the primitive sheath. 



Medullary segments. Other breaks of continuity are seen in the 

 medullary sheath (figs. 353, 361) which are of an entirely different nature 

 from the nodes of Ranvier ; indeed it is somewhat uncertain how far they 

 correspond to a pre-existent structure in the fibre. In consequence of 

 their presence the medullary sheath appears as if made up of a number 

 of small cylindrical segments with either conical or funnel-shaped 

 ends which fit in with one another in the alternate segments. The 

 segments in question have been frequently described as integral 

 constituents of the medullary sheath (Schmidt, Lantermann and others). 

 It is easy to convince oneself of the reality of the appearances here 

 mentioned, but it is far less easy to be certain that they are not artificial 

 productions. Against the view of their existence in the natural condi- 

 tion it is to be noted that they are extremely variable in number and in 

 size in a given length, even of the same nerve-fibre, that they appear 

 to become increased in number if the nerve-fibre have been subjected to 

 much manipulation, that they have no constant relation, so far as can 

 be made out, to the other parts of the medullated fibre, and that, 

 according to the testimony of several careful observers, they are not to 

 be seen in the nerve-fibres of the living animal, unless these have been 

 subjected to an abnormal amount of traction or other mechanical injury. 

 This last assertion is denied, however, by others, who maintain the 

 pre-existence of the medullary segments, and describe them and the 

 oblique clefts which separate them as definite parts of the sheath. 

 Kanvier on the other hand, who first described the original protoplasmic 

 condition of the medullary sheath, considers that the protoplasm has 

 disappeared and become replaced by the pseudo-fatty substances (lecithin, 

 neurin, &c.), except next to the primitive sheath or neurolemma, and next 

 to the axis-cylinder, where fine layers still remain, and he regards these as 

 being connected with one another by protoplasmic septa, which lie in the 

 intervals between the conico-cylindrical segments of the medullary sheath. 

 In short he regards each segment of the medullary sheath as repre- 

 senting an elongated cell occupied mainly by this fatty material and 

 wrapped round the axis-cylinder. 



Careful examination of the fibres in the fresh condition reveals fine 

 obliquely disposed lamellae, which in optical section have the appearance of 

 fibres, bridging across the clefts between the segments (fig. 353). In nerve- 

 fibres which have been treated by a mixture of bichromate of potash and 

 osmic acid, and afterwards by nitrate of silver, the situation of each cleft 

 is occupied by what looks like a thread of darkly stained substance passing spirally 

 around the fibre (Rezzonico, Golgi). The meaning of these appearances is by no 

 means clear. 



Bod-like and reticular structures in the medullary sheath. It was 



