THE STRUCTURE OF THE NERVE SYSTEM 337 



tissue. The latter contains the bloodvessels and 

 lymphatics. Nerves are the connecting trunks which 

 bring the brain and spinal cord in relation with the 

 structures throughout the body muscles, skin, glands, 

 etc. 



Structure of Nerves. Examined under the micro- 

 scope, each nerve shows on cross-section a collection 

 of nerve fibers, or axones, arranged in bundles. The 

 entire group being invested by a layer of connective 

 tissue called the epineurium; however, each separate 

 bundle of fibers within the nerve is enclosed in its 

 own connective-tissue sheath the perineurium, and 

 the numbers of axis-cylinders, or axones, within the 

 bundles are again enclosed in a delicate connective- 

 tissue net- work called the endoneurium, or sheath of 

 Henle. 



Nerves as they pass from the brain or spinal cord 

 are surrounded by the epineurium, and divide and 

 subdivide as they give off branches to other nerves, and 

 pass to their terminations. As each branch is given 

 off the same sheath of connective tissue that enclosed 

 the parent nerve accompanies it. This arrangement 

 continues as the branches become smaller and smaller, 

 until they reach their terminal point of distribution, 

 where only a single nerve fiber remains, enclosed by a 

 transparent membrane, the endoneurium or sheath 

 of Henle. However, near the ultimate termination 

 of a nerve the single nerve fiber may continue to give 

 off branches, each one consisting of the axis-cylinder 

 and myelin sheath. 



The multitude of nerve cells or neurones entering 

 into the formation of the nerve system are supported 

 in a non-neural or inactive set of cells as regards nerve 

 activity, called the supporting tissue elements of the 

 nerve system. These consist of two kinds : (1) neuroglia, 

 and (2) connective-tissue trabeculse derived from the pia 

 mater, or the bloodvessel channels, Neuroglia consist 

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