ioo6 



HUMAN ANATOMY. 



The Nerve-Trunks. — The fibres composing the peripheral nervous system are 

 grouped into the larger and smaller nerve-trunks which extend to various parts of the 

 body. In the make-up of those that supply both muscles and sensory surfaces 

 (integument or mucous membranes), as, for example, the median or the third division 

 of the trigeminal nerve, three sets of fibres are included : ( i ) the efferent axones of 

 motor neurones whose cell-bodies are situated within the spinal cord or brain ; (2) the 

 afferent dendrites of sensory neurones within the spinal and other sensory ganglia ; 

 and (3) the efferent axones of neurones within the sympathetic ganglia that accompany 

 the spinal fibres to the periphery and serve for the innervation of the involuntary 

 muscle of the blood-vessels and of the skin and the glands. 



The nerve-fibres, the various kinds usually more or less intermingled, are 

 grouped into bundles, the funiculi, which differ in number and diameter according 

 to the size of the entire trunk that they form. Each funiculus is surrounded by a 

 definite sheath of dense connective tissue, the permeurijaji, which is directly con- 

 tinuous with the delicate fibro-elastic tissue prolonged between the individual nerve- 

 fibres as the cndoneitrium. When well represented, the sheath of the funiculus 

 consists of concentric lamellae of fibrous tissue which enclose perhieurial ly77iph- spaces. 



Fig. 849. 



Epineurium 



Blood-vessels 



Perineurium 



Transverse section of small nerve-trunk composed of loosely united funiculi. X 20. 



Funiculus of 

 nerve- fibres 



The latter, lined by flattened connective-tissue plates, are in relation with the clefts 

 between the nerve-fibres, on the one hand, and with the lymphatics within the inter- 

 funicular tissue on the other. Where, as usual, the nerve is composed of several 

 funiculi, these are loosely bound together and the entire trunk so formed is invested 

 by a general fibro-elastic envelope, the epineiirium, in which course the blood-vessels 

 and lymphatics. These envelopes of the nerve-trunk are continued over its branches, 

 even onto its smallest subdivisions. The last representative of these coverings 

 is seen on the individual fibres as the sheath of Henle, that surrounds the fibre 

 and consists of flattened cells and delicate strands of connective tissue outside the 

 neurilemma. 



In cross-sections of the nerve-trunk (Fig. 850), the transversely cut individual 

 medullated nerve-fibres appear as small circles, sharply defined by a fine outline (the 

 neurilemma), each enclosing a deeply stained dot (the axis-cylinder in section). 

 The interval between the latter and the neurilemma, corresponding to the space 

 occupied by the myelin, usually appears clear and unstained with the exception of 

 delicate and uncertain suggestions of membranous septa. In contrast with its 

 unstained appearance in sections tinged with carmine, after the action of osmic acid 

 or special hematoxylin staining (Weigert) the medullary substance exhibits a dark 

 color and the axis-cylinder appears surrounded by a deeply tinted ring. The neuri- 



