OF THE NERVOUS SYSTEM. 



405 



finally the whole are united into a single compound mass by its exterior 

 investment, which is known as the u neurilemma." Such a complete 

 bundle is called a nerve, and the nerve fibres of which it is composed 

 are usually all distributed, after a longer or shorter transit, to associated 

 organs, or to adjacent regions of the body. 



The nerve fibres themselves are not known to divide, branch, or inoscu- 

 late with each other in any part of their course through the main trunks 

 and branches of the nerves. So far as observation goes, each nerve 

 fibre is continuous and independent, from its origin in the nervous 

 centres to within a microscopic distance of its peripheral termination. 

 When a nerve therefore divides during its course into several branches, 

 or when the branches of adjacent nerves inosculate with each other 

 to form a plexus, like the cervical, brachial, or lumbar plexuses, this is 

 only because certain ultimate nerve fibres, or bundles of fibres, leave 

 those with which they were previously associated, and pursue a different 



Fig. 136. 



Fig. 137. 



DIVISION OF A NKRVOUS BRANCH 

 (a), into its ultimate fibres, 6, c, d, e. 



Inosculation of NERVES. 



direction. A nerve which originates, for example, from the spinal cord 

 and runs down the upper extremity, to be finally distributed to the in- 

 tegument and muscles of the hand, contains at its point of origin all the 

 filaments into which it is afterward divided, and which are merely sepa- 

 rated at successive points from the main bundle. Jn case of the inoscu- 



