254 THE NERVOUS SYSTEM 



interrupted investment. At the nodes, when the medullary 

 sheath is deficient, it is contiguous with the axis cyhnder. 

 The neurolemma is only found in nerve-fibres outside the 

 central nervous system. 



The above description appUes equally to dendrons and 

 to axons. These differ in that dendrons branch very 

 freely, while axons, though they give off minute lateral 

 branches {axo7i collaterah), do not properly divide until 

 near their termination. 



Under physiological conditions an impulse travels from 

 the dendrons to the cell-body and from the cell-body to 

 the axon (axi petal conduction). 



Non-medullated fibres, as their name implies, have no 

 medullary sheath. Upon them he nuclei beheved to belong 

 to a kind of neurolemma. They ramify more freely than 

 medullated fibres. 



In the nerve -trunk the nerve-fibres are packed together 

 in bundles, which are separated by connective tissue — the 

 perineurium. This contains blood-vessels, lymphatics and 

 sensory nerve-endings (nervi nervorum). Surrounding it 

 is an outer fibrous layer — the epineurium. 



Degeneration and Regeneration of Nerve 



When a nerve is cut, the axons which are separated 

 from their cell-bodies undergo the process of Wallerian 

 degeneration. The medullary sheath is decomposed into a 

 mass of fatty globules, and the axis cylinder becomes broken 

 into fragments. The dMris is absorbed by leucocytes. 

 Meanwhile the nuclei of the sheath of Schwann undergo 

 proliferation, forming a chain of cells in which fibres are 

 deposited. Regeneration — a slow process — occurs by down- 

 growth of fibres from the central stump. These find their 

 way into the peripheral part, the newly formed fibres of 

 which form a kind of scaffolding for the new nerve pro- 

 cesses to grow down. 



When a motor nerve is cut profound changes take place 

 in the muscle which it supplies. There is a high degree 



