CHAP. IL] THE CONTRACTILE TISSUES. 117 



it undergoes is that, in many if not all cases, it divides near its 

 termination in the tissue, and in some cases the divisions are 

 numerous and join or anastomose freely. Obviously the axis 

 cylinder is the essential part of the nerve fibre. 



2. The primitive sheath or neurilemma, a tubular sheath of 

 transparent apparently homogeneous material, not unlike that of 

 a sarcolemrna in nature. At each node the neurilemma is con- 

 stricted so as to embrace the axis cylinder closely, but is at the 

 same time thickened by some kind of cement material. Staining 

 reagents, especially silver nitrate, appear to enter the nerve fibre 

 from without more readily at a node than elsewhere, staining 

 the fibre most at the node, and creeping upwards and downwards 

 from the node along the axis cylinder ; hence it has been supposed 

 that the nutritive fluid, the lymph, enters into the fibre and so 

 gets access to the axis cylinder more readily at the nodes than else- 

 where. About midway between every two nodes is placed a long 

 oval nucleus, on the inside of the neurilemma, pushing the 

 medulla, as it were, inwards, and so lying in a shallow bay 

 of that substance. Immediately surrounding the nucleus is a 

 thin layer of granular substance of the kind which we have spoken 

 of as undifferentiated protoplasm ; in young newly formed fibres at 

 all events and possibly in all fibres a very thin layer of this same 

 substance is continued all over the segment between the nodes, on 

 the inner surface of the neurilemma between it and the medulla. 



3. The medulla. This is a hollow cylinder of fatty material 

 of a peculiar nature filling all the space between the neurilemma 

 on the outside and the axis cylinder within, and suddenly ceasing 

 at each node. It thus forms a close fitting hollow jacket for the 

 axis cylinder between every two nodes. The fatty material is 

 fluid, at least at the temperature of the body, but appears to be 

 held in its place as it were by a network of a substance called 

 neurokeratin, allied to the substance keratin, which is the basis of 

 the horny scales of the epidermis and of other horny structures ; 

 this network is most marked towards the outside of the medulla. 



So long as the nerve is in a fresh living, perfectly normal 

 condition, the medulla appears smooth and continuous, shewing no 

 marks beyond the double contour; but in nerves removed from 

 the body for examination (and according to some observers, at 

 times in nerves still within the body) clefts make their appearance 

 in the medulla running obliquely inwards from the neurilemma to 

 the axis cylinder, and frequently splitting up the medulla in such 

 a way that it appears to be composed of a number of hollow cones 

 partially slid one over the other along the axis cylinder. These 

 clefts are spoken of as indentations. At a later stage of alteration 

 the medulla may divide into a number of small irregular masses 

 separated by fluid; and since each small piece thus separated has a 

 double contour, like a drop of medulla exuded from the end of a 

 fibre, the whole fibre has an irregular ' curdy ' appearance. 



