CHAPTEE XH. 

 \ 

 ANATOMY OF THE NERVOUS SYSTEM. 



Nerve-Trunks. In dissecting the Human Body numerous 

 white cords are found which at first sight might be taken for 

 tendons. That they are something else however soon becomes 

 clear, since a great many of them have no connection with 

 muscles at all, and those which have usually enter somewhere 

 into the belly of the muscle, instead of being fixed to its ends 

 as most tendons are. These cords are nerve-trunks : followed 

 in one direction each (Fig. 69) will be found 'to break up into 

 finer and finer branches, until the subdivisions become too 

 small to be followed without the aid of a microscope. Traced 

 the other way the trunk will in most cases be found to in- 

 crease by the union of others with it, and ultimately to join 

 a much larger mass of different structure, from which other 

 trunks also spring. This mass is a nerve-centre. That end 

 of a nerve attached to the centre is naturally its central, 

 and the other its distal or peripheral end. Nerve-centres, 

 then, give origin to nerve-trunks; these latter spread all over 

 the Body, usually branching and becoming smaller and smaller 

 as they proceed from the centre; they finally become very 

 small, and how they ultimately end is not in__alLcas_es certain, 

 but it is known that some have sense-organs at their termina- 

 tions and others muscular fibres. The general arrangement 

 of the larger nerve-trunks of the Body is shown in Fig. 69. 

 Physically a nerve is not so tough or strong as a tendon of 

 the same size; it may readily be split up into longitudinal 

 strands, each of which consists of a number of microscopic 

 threads, the nerve-fibres, bound together by connective tissue. 



Plexuses. Very frequently several neighboring nerve- 

 trunks send off communicating branches to one another, each 

 branch carrying fibres from one trunk to the other. Such 

 networks are called plexuses (Fig. 72), and through the inter- 

 changes taking place in them it often happens that the distal 



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