NERVE FIBRES AMD NERVES si 7 



The bloodvessels supplying a nerve terminate in a minute capillary plexus, the vessels com- 

 posing which pierce the perineurium and run, for the most part, parallel with the fibres; they 

 are connected by short, transverse vessels, forming Harrow, oblong meshes, similar to the capillary 

 system of muscle. Fine amyelinic axones accompany these capillary vessels, the vasomotor 

 fibres, and break up into elementary fibrils, which form a network around the vessel. Horsley 

 has also demonstrated certain myelinic fibres as running in the epineurium and terminating 

 in small bulboid tactile corpuscles or end-bulbs of Krause. These nerve fibres, believed to 

 be sensor, and termed nervi nervorum, are considered to have an important bearing upon 

 certain neuralgic pains. 



Nerves, in their course, subdivide into branches, and these frequently communicate with 

 branches of a neighboring nerve. 



The axones, so far as is at present known, do not coalesce, but pursue an uninterrupted course 

 from the centre to the periphery. In separating a nerve, however, into its component fasciculi, 

 it may be seen that they do not pursue a perfectly insulated course, but occasionally join at a 

 very acute angle with other fasciculi proceeding in the same direction; from this branches are 

 given off, to join again in like manner with other fasciculi. It must be distinctly understood, how- 

 ever, that in these communications the axones do not coalesce, but merely pass into the sheath 

 of the adjacent nerve, become intermixed with its axones, and again pass on, to become blended 

 with the axones in some adjoining fasciculus. 



The communications which take place between two or more nerves form what is called a 

 plexus. Sometimes a plexus is formed by the primary branches of the trunks of the nerves 

 as the cervical, brachial, lumbar, and sacral plexuses and occasionally by the terminal fasciculi, 

 as in the plexuses formed at the periphery of the body. In the formation of a plexus the com- 

 ponent nerves divide, then join, and again subdivide in such a complex manner that the indi- 

 vidual fasciculi become interlaced most intricately; so that each branch leaving a plexus may 

 contain filaments from each of the primary nerve-trunks which form it. In the formation also 

 of smaller plexuses at the periphery of the body there is a free interchange of the fasciculi and 

 primitive fibres. In each case, however, the individual filaments or axones remain separate and 

 distinct. 



It is probable that through this interchange of fibres, every branch passing off from a plexus 

 has a more extensive connection with the spinal cord than if it had proceeded to its distribution 

 without such connections with other nerves. Consequently the parts supplied by these nerves 

 have more extended relations with the nerve centres; by this means, also, groups of muscles 

 may be associated for combined action, as is best exemplified in the formation of the limb 

 plexuses. 



The sympathetic nerves are constructed in the same manner as the cerebrospinal nerves, 

 but consist mainly of amyelinic axones, collected into fasciculi and enclosed in a sheath of con- 

 nective tissue. There is, however, in these nerves a certain admixture of myelinic axones, 

 and the amount varies in different nerves, and may be known by their color. Those branches 

 of the sympathetic which present a well-marked reddish-gray color are composed more especially 

 of amyelinic axones, intermixed with a few myelinic axones; while those of a white color contain 

 more of the latter and a few of the former. Occasionally, the gray and white cords run together 

 in a single nerve, without any intermixture, as in the branches of communication between the 

 sympathetic ganglia and the spinal nerves, or in the communicating cords between the ganglia. 

 The nerves, both of the cerebrospinal and sympathetic systems, convey impressions of a two- 

 fold kind. The afferent or centripetal nerves, generally called sensor, transmit to the nerve 

 centres impressions made upon the peripheral ends of their components, to produce reflexes 

 in the lower centres while the mind, through the medium of the brain, becomes conscious of 

 environmental conditions or changes. The efferent or centrifugal (in large part "motor") 

 nerves transmit impulses from the centres to the parts to which the nerves are distributed; these 

 impulses either excite muscle contraction or influence the processes of nutrition, growth, and 

 secretion. 



The ganglia may be regarded as separate small aggregations of nerve cells, 

 Connected with each other, with the cerebrospinal axis, and with the nerves in 

 various situations. They are found on the dorsal root of each of the spinal nerves ; 

 on the sensor root of the trigeminus; on the facial and auditory nerves; and on 

 the glossopharyngeal and vagus nerves. They are also found in a connected 

 series along each side of the vertebral column, forming the gangliated cord or 

 trunk of the sympathetic; and on the branches of that nerve, generally in the 

 plexuses or at the point of junction of two or more nerves with each other. or 

 with branches of the cerebrospinal system. On section they are seen to consist 

 of a reddish-gray substance, traversed by numerous white nerve fibres; they vary 



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