460 ELEMENTARY PHYSIOLOGY 



may be readily seen as little circular dots arranged 

 in groups whicli compose the larger part of the 

 section. These fibres are bound together in bundles, 

 which are rounded as seen in section, by an external 

 sheath or case of connective tissue called the peri- 

 neurium, from whose imier surface verj' delicate layers 

 of connective tissue pass in between the fibres of which 

 each bundle is composed. The several bundles are 

 themselves bound together by connective tissue to form 

 the trunk of the nerve, and the whole nerve, thus built 

 up of bundles of nerve-fibres, is surrounded and held 

 together by an external layer of connective tissue. 



The nerve-fibres, which are the essential elements of 

 the nerve, vary in diameter from 2/n to 12/x or more. In the 

 living state they are very soft cylindrical rods of a glassy, 

 rather strongly refracting aspect. No limiting membrane 

 is distinguishable from the rest of the substance of the 

 rod, but running through the centre of it a band of some- 

 what less transparency than the rest may be discerned. 

 At intervals, the length of which varies, but is always 

 many times greater than the thickness of the rod, the 

 nerve fil)re presents sharp constrictions, which are termed 

 nodes (Fig. 146, A, n ; B. n it). Somewhere in the inter- 

 space between every two nodes, very careful examination 

 will reveal the existence of a nucleus (Fig. 146, B. ?i, c), 

 invested by more or less protoplasmic substance and 

 lying in the substance of the rod, but close to the 

 surface. 



As the fibre dies, and especially if it is treated with 

 certain re-agents, these appearances rapidly change. 

 1. The outermost layer of the fibre becomes recognisable 

 as a definite membrane, the neurilerama ^ (the so-called 



1 This word was formerly used to denote the whole nerve-case, now 

 called perineurium ; but its similarity to the word sarcolemvia led to 

 great confusion in the minds of students. ' It is undoubtedly a whole- 

 some rule never to use an old word in a new sense ; but the striking 

 similarity between the two words "neurilemma" and " sarcolemma," 

 and between the nerve-fibre sheath and the muscle-fibre sheath, seems an 

 adequate excuse for an exception to the rule. 



