THE STRUCTURE OF THE ELEMEHTARY TISSUES. 95 



Nodes of Ranvier. At regular intervals in most medullated nerves 

 the nucleated sheath of Schwann possesses annular constrictions; these 

 lire called nodes of Ranvier. At these points (fig. 97), the contin- 

 uity of the medullary white substance is interrupted, and the primitive 

 sheath comes into immediate contact with the axis-cylinder. The seg- 

 ment of the fibre between two nodes is termed an internode, and the 

 length of the interned es varies in different nerves; their average is said 

 to be 1 mm. There is only one nerve nucleus to each internode. At 

 each node the internodes are united within the external sheath by a 

 band, constricting band of Ranvier (fig. 101), and this stains black with 

 silver nitrate; the axis-cylinders at the nodes also are capable of being 



Fig. 99. Transverse section of sciatic nerve of the rabbit, hardened in chromic acid and 

 stained with picro-carmine, and showing lamellar sheath, peripheric connective tissue, and intra- 

 fascicular connective tissue. X 550 and reduced one-half, a, Perifusoicular connective tissue; 6, 

 lamellar t-heath; c, iutra-fascicular connective tissue; d, nerve-fibre cut across, showing nuclei of 

 the same; e, axis-cylinder. 



stained with the same reagent, and so a node-of Ranvier when stained 

 with silver nitrate is marked by a black cross. 



Size. The size of the nerve-fibres varies (fig. 99); it is said that 

 the same fibres may not preserve the same diameter through their whole 

 length. The largest fibres are found within the trunks and branches of 

 the spinal nerves, in which the majority measure from 14.4/* to 19/* in 

 diameter. In the so-called visceral nerves of the brain and spinal cord 

 medullated nerves are found, the diameter of which varies from 1.8,u to 

 3.6/jL. In the hypoglossal nerve they are intermediate in size, and gene- 

 rally measure 7.2fi to 10.8/Jt. 



Non-medullated Fibres. The fibres of the second kind (fig. 98) 

 which are also called fibres of RemaJc, constitute the principal part of 

 the trunk and branches of the sympathetic nerves, the whole of the 



