RELATIONS OF NEURONES 



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fibre. Efferent fibres which bear impulses to skeletal muscle are known as somatic efferent 

 fibres, while those which terminate upon the cell-bodies of sympathetic neurones and thus bear 

 impulses destined for smooth muscle, cardiac muscle and glands (secretory) are visceral or 

 splanchnic efferent fibres. 



A nerve is a closely associated aggregation of parallel nerve fibres coursing in the periphery. 

 It may be spinal, cranial or sympathetic according to its attachment or according to the origin 

 of the majority of its fibres. It may contain several functional and structural varieties of fibres. 

 The spinal nerves contain all structural varieties. Nerve roots are those bundles of fibres which 

 join to form a nerve. Most of the cranial nerves have but one root of origin. Nerve roots, in 

 their turn, are formed by the junction of smaller root-filaments. Nerve branches result from 

 the division of the nerve, the separation of its component fibres into separate bundles. Some 

 branches are of sufficient size and significance to be called nerves and given separate names. 

 The smaller branches are called rami, twigs, etc. 



In the central system, a given bundle of fibres is called a fasciculus, while two or more adja- 



FiG. 611. 



-Diagram of Transverse Section of Medulla Oblongata, Illustrating Nuclei 

 OF Termination and Nuclei of Origin. 



Nucleus of termination 

 of vagus 



Nucleus of termination 

 of vestibular 



Nucleus of origin of motor 

 portion of vagus 



Root ganglion of vagus 

 1 



I 



cent fasciculi coursing parallel to each other comprise a funiculus, a bundle of bundles. The 

 central nervous system is bilaterally symmetrical throughout its length. A bundle of fibres 

 arising from cell-bodies situated on one side and crossing the mid-line transversely to terminate 

 in the opposite side is a commissure. The commissures vary greatly in size and contain fibres 

 crossing in both directions. Scattered fibres which cross the mid-line are commissural fibres. 

 Fibres of varying length, arising from cell-bodies situated in one locality of the central system, 

 which do not cross the mid-line, but terminate in other localities of the same side, above and 

 below the level of their origin or in a different region of the same level, form association fasciculi. 

 The shortest association fasciculi, not extending beyond the bounds of a given division of the 

 central system, are known as fasciculi proprii. When bundles of the same origin, functional 

 direction and significance, running one on either side of the mid-line, cross the mid-line they 

 are said to decussate and the crossing is known as a decussation. In the decussations, the direc- 

 tion of the crossing is oblique rather than transverse. 



The cell-bodies of neurones whose axones go to form certain nerve roots, fasciculi and certain 

 commissures show a tendency to accumulation in localized masses. In the peripheral system, 

 such an accumulation of cell-bodies is known as a ganglion; in the central system such is distin- 

 guished as a nucleus. Thus, there are the sympathetic ganglia which give rise to sympathetic 

 nerves and sympathetic roots of nerves; and on the beginning of each spinal nerve there is a 

 spinal ganglion which gives rise to the afferent fibres of its dorsal root and in its nerve trunk. 

 There are ganglia on the cranial nerves which give rise to the afferent or sensory axones in them 

 and which are of the same significance as the spinal ganglia. Every ganglion, therefore, has 



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