TWENTIETH LECTURE. 



THE ARRANGEMENT AND TERMINATION OF THE NERVE 



FIBRES. 



The spinal nerves and those of the brain appear white 

 through the medullary sheaths of their tubular constituents ; 

 the trunks of the sympathetic appear gray from the excess 

 of non-medullated fibres. 



The former, at their exit from the central organ, become 

 invested in a delicate connective-tissue envelope ; they are 

 subsequently surrounded by an additional reinforcement of 

 connective tissue, furnished by the dura mater. This affords 

 together the nerve sheath, perineurium or neurilemma. This 

 connective tissue penetrates, in a lamellar or sheath-like man- 

 ner, between the bundles of nerve fibres, becoming, at the 

 same time, looser and softer. Its modified boundary layer 

 forms at last the primitive sheath of the nerve tube. A 

 scanty, straight net- work of finest capillary vessels permeates 

 the whole. Injections made from the lymph spaces likewise 

 penetrate beneath the perineurium and between the nerve 

 bundles (Key and Retzius). 



The primitive fibres run alongside of each other in the 

 nerve trunk, undivided and indifferent. The nerve trunks 

 usually send off their branches at an acute angle, the bundles 

 of fibres bending away from the main path to the lateral. 



When anastomoses take place, groups of fibres pass, at the 

 point of communication, from the one nerve to the other, or 

 we have a double interchange of fibres. 



The perineurium becomes finer and finer in proportion as 

 we proceed from the larger trunks to the finest systems of 

 branches. Finally, it appears as a striated or more homo- 

 geneous connective substance with rather stunted cells. 



