1 68 THE CEREBRUM. 



drites. If long they become medullated. Their conduction is 

 toward the cell-body, cellulipetal, or afferent. Axones, neurax- 

 ones or axis-cylinders (Fig. 50) are smooth and fibrillar in charac- 

 ter. One (occasionally two or more) is given off from the axone- 

 hillock of the cell-body. It may break up into branches at once, 

 constituting a dendraxom; but usually it runs a considerable 

 distance and sends out side branches, called collaterals. When 

 long, as a rule, it becomes medullated as its function develops, and 

 the medullary sheath presents segmental divisions separated by 

 constrictions, called the nodes of Ranvier, at which points the col- 

 laterals are given off. Both the parent axone and all its collaterals 

 end by multiple division in the form of an end-brush, or tassel. 

 This tassel lies in contact with the dendrites or body of another 

 neurone, with a secreting cell, or with a muscle fiber. In the last 

 instance the fibers of the tassel spread out into disc-like plate- 

 lets, called motorial end-plates (Fig. 50). Axone conduction is 

 from the cell-body, cellulifugal, or efferent. 



The myelin sheath of axones and dendrites, which is devel- 

 oped as the neurone begins to functionate, is imbedded in neurog- 

 lia within the optic nerve and tracts, and in the brain and spinal 

 cord ; but, elsewhere, is surrounded by the neurolemma (Schwann) 

 and the -fibrous sheath of Henle. The fibers of the olfactory nerve 

 and most sympathetic fibers are not medullated, but they possess 

 the fibrous sheath. Near the cell-body and near the end-tuft 

 the processes are naked, having neither the fibrous nor the medul- 

 lary sheath. 



Types of Neurones. i. The first type has a long axone, which 

 preserves its identity, though it may give off many collaterals. 

 Found in brain and spinal cord (Deiters). 



2. The second type has a short axone, breaking at once into 

 branches of apparently equal importance, the dendraxone. Found 

 in cerebrum and cerebellum (Golgi). 



3. The third type has two or more axones diaxone, triaxone, 

 polyaxone as in the first layer of the cerebral cortex (Cajal). 



Orders of Neurones. i. The first order has distal process 

 in relation with the periphery, as spinal-ganglion and anterior 

 columna neurones, and conducts from the periphery or to it. 



