118 



THE NEKVOUS TISSUES 



either entirely absent or, at most, very insignificant in these fibres. 

 The neurilemma is perhaps incomplete at times, but exhibits fre- 

 quent nuclei along the course of the fibre. Fibres of this type 

 are of quite frequent occurrence in the cranial nerves of the cere- 

 bro-spinal system. Other cerebro-spinal nerve fibres lose their 

 medullary sheath and finally also their neurilemma prior to their 

 termination. 



4. Non-medullated Nerve Fibres without a Neurilemma. These 

 fibres are naked axis cylinders and as such are found at the cyto- 

 proximal end of the neuraxis in the grey matter of the central 

 nervous system, and at the cytodistal end prior to the termination 

 of the neuraxis in its arborization of terminal fibrils. In man 

 nerve fibres are of this type throughout their entire course, only in 

 the olfactory nerves. 



NERVE TRUNKS. The nerve fibres of the peripheral nervous 

 system are united into bundles to form the nerve trunks or nerves. 



Each nerve is sur- 

 b rounded by a heavy 



connective tissue 

 sheath, the epineu- 

 rium, which sends 

 trabeculum-like sep- 

 ta into the nerve. 

 These septa subdi- 

 'vide the nerve trunk 

 into smaller bundles 

 of nerve fibres, the 

 funiculi. The fu- 

 niculus forms a 

 compact bundle of 

 nerve fibres, and is in 

 turn invested with 

 a sheath of dense 

 connective tissue, 



the perineurium. Hence the perineurium stands in the same re- 

 lation to the funiculus as does the epineurium to the whole nerve 

 trunk. 



From the inner surface of the perineurium, septa pass into the 

 funiculus and break up to form a fine connective tissue framework 

 between the individual nerve fibres. Thus the nerve fibres are 

 embedded in a delicate connective tissue sheath. On teasing the 



FIG. 114. TRANSECTION OF THE SCIATIC NERVE OF A DOG. 



The fat cells and the myelin sheaths of the nerve fibres 

 have been blackened by osmium tetroxid. a, fat cells ; 

 6, &', blood vessels, that at &' lies within a funiculus ; c, 

 epineurium ; d, perineurium ; e, coarser bands of the endo- 

 neurium. Osmium tetroxid. Photo, x 30. 



