io6 ELEMENTS OF HISTOLOGY. LChap. 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- 



64. Transv'erse Section through a Nerve-bundle 

 in the Tail of Mouse. 



P, The perineuriuni ; , the endoneurium separating the medullated nerve-fibres 

 seen in cross section ; 7, lymph spaces in the perineurium ; V, 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 lymphatica 

 magna in the frog, receive from the serous membrane 

 an outer endothelial covering. 



139. The nerve-fibres in the nerve-bundles of the 

 cerebro-spinal nerves, with the exception of the 

 olfactory nerve, are medullated nerve-fibres. These are 

 doubly or darkly contoured smooth cylindrical fibres, 

 varying in diameter between ^TrW and y^^u of 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- 



