Chap. II.] 



A NERVOUS SYSTEM. 



31 



The ultimate bundles of elementary * fibrils ' are 

 gradually aggregated- into larger bundles, or 'fibres,' as 

 they recede from their seats of origin or termination and 

 approach the nerve centres with which they are in com- 

 munication. These smaller bundles soon become enve- 

 loped in a very delicate membranous sheath (Schwann), 

 whilst the component fibrils fuse more or less completely, 

 so that the fibre appears either 

 structureless (fig. 3), or merely 

 shows signs of fibrillation. A little 

 further on these still small fibres 

 become enveloped by a layer of 

 white semi-fluid ' medullary sub- 

 stance,' which lies beneath the 

 membranous sheath of Schwann, 

 and forms a white border to the 

 nerve as it is seen on micro- 

 scopical examination. Thus a 

 dark borxlered, ivhite, or medid- 

 lated nerve fibre is formed. 



Such dark-bordered fibres are 

 at first very slender ; but by co- 

 alescence with others of the same 

 kind larger fibres are produced 

 (fig. 4), varying in man from x'ilooo^^^ *^ T o~o"o^^ ^^ ^^ 

 inch in diameter. The central portion of such a nerve 

 fibre, viz., that lying within the white medullary sheath, 

 is its most important constituent ; it is almost translucent, 

 and is known as the axis band or axis cylinder. In 

 the perfectly fresh state it shows faint traces of fibril- 

 lation, but unless examined with care it may api3ear 

 structureless, and yield no evidence to the microscopic 

 observer as to its compound nature. Under the influence 

 of slight traction, or by imbibition of water, these medui- 



Fio. 3. — Human Nerve Fibres 

 of different sizes (Kolliker). 



a, a, a. Healthy fibres, the 

 largest of which is ' dark-bor- 

 dered.' b, b. Fibres altered by 

 exposure. Magnified 350 dia- 

 meters. 



