io6 ELEMENTS OF HISTOLOGY. [Chap. xiv. 



When nerve-trunks anastomose so as to form a 

 plexus e.g. , in the brachial, or sacral plexus there 

 occurs an exchange and re- arrangement of nerve- 

 bundles in the branches. A similar condition obtains 

 in the ganglia of the cerebro-spinal nerves. Nerve- 



Fig. 64. Transverse Section through a Nerve-bundle 

 in the Tail of Mouse. 



V, The perineuriuni ; e, the endoneurium separating the medullated nerve-fibres 

 seen in cross section ; I, lymph spaces in the periueurium ; I, lymph spaces 

 in the endoneurium. (Atlas.) 



trunks and nerve-branches passing through a lymph 

 cavity, such as the subdural spaces, or the sub- 

 cutaneous lymph sacs, or the cisterna lyrnphatica 

 magna in the frog, receive from the serous membrane 

 an outer endothelial covering. 



139. The ultimate nerve-fibres in the funiculi of 

 the cerebro-spinal nerves, with the exception of the 

 olfactory nerve, are medullated nerve-fibres. These 

 are doubly contoured smooth cylindrical fibres, 

 varying in diameter between ^innr an d T^Jrnr ^ an 

 inch. Within the same bundle of a nerve e.g., of 

 the brachial or sacral plexus there occur fibres which 

 are several times thicker than others, and it is pro- 

 bable that they are derived from different sources. 

 Schwalbe has shown that the thickness of the nerve- 



