cxliv NERVOUS SYSTEM. 



Courvoisier describes both fibres as acquiring a medullary sheath, the straight one 

 first. He has found the above described structure in the ganglia of fish, birds, and 

 mammals ; but whilst in the frog the cell has never, or scarcely ever, more than one 

 straight and very rarely more than one spiral fibre, he finds that in other vertebrates 

 a cell may give off such twin fibres from two or more parts of its circumference. 



In the spinal ganglia of the skate, torpedo, and dog-fish, there is a different arrange- 

 ment. In these, as first pointed out by R. Wagner, two fibres are connected with 

 each ganglion-cell, at opposite sides or opposite poles, one directed centrally toward 

 the root of the nerve, and the other outwardly towards its branches. 



CEREBKO- SPINAL NERVES. 



These are formed of the nerve-fibres already described, collected together 

 and bound up in sheaths of connective tissue. A larger or smaller number 

 of fibres inclosed in a tubular sheath form a slender round cord of 110 deter- 

 minate size, usually named a funiculus ; if a nerve be very small it may 



consist of but one such cord, 



rig. .. . . . , ., 



but in larger nerves several fum- 

 culi are united together into 

 one or more bundles, which, 

 being wrapped up in a com- 

 mon membranous covering, con- 

 stitute the nerve (fig. LXXXIV.). 

 Fig. LXXXIV. -PORTION OF THE TRUNK OF A NERVE Accordillg i y in dissecting a 



CONSISTING OF MANY SMALLER CORDS OR FUNICULI 



WRAPPED UP IN A COMMON SHEATH. nerV6 > W6 nrsfc COme t0 ftl1 OUt ' 



., i r T j <. ward covering, formed of con- 



A, the nerve ; B, a single funiculus drawn out 



from the rest (from Sir C. Bell). nective tissue, often so strong 



and dense that it might well be 



called fibrous. From this common sheath we trace laminae passing inwards 

 between the larger and smaller bundles of funiculi, and finally between the 

 funiculi themselves, connecting them together as well as conducting and sup- 

 porting the fine blood-vessels which are distributed to the nerve. But, besides 

 the interposed areolar tissue which connects these smallest cords, each funi- 

 culus has a special sheath of its own, as will be further noticed presently. 



The common sheath and its subdivisions consist of connective tissue, pre- 

 senting the usual white and yellow constituent fibres of that texture, the 

 latter being present in considerable proportion. The special sheaths of the 

 funiculi, on the other hand, appear to be formed essentially of a fine trans- 

 parent membrane, which may without difficulty be stripped off in form of a 

 tube from the little bundle of nerve-fibres of which the funiculus consists. 

 When examined with a high power of the microscope, this membrane 

 presents the aspect of a thin transparent film, which in some parts appears to 

 be quite simple and homogeneous, but is more generally marked with 

 extremely fine reticulated fibres. Corpuscles resembling elongated cell- 

 nuclei may also be seen upon it when acetic acid is applied. The tissue in- 

 vesting a nerve and inclosing its proper fibres, as now described, is named 

 the neurilemma, and the term is for the most part applied indiscriminately 

 to the whole of the enveloping structure, though some anatomists use it to 

 denote only the sheaths of the funiculi and smaller fasciculi, whilst they 

 name the general external covering of the nerve its *' cellular sheath. " (vagina 

 cellulosa). 



Some recent writers, believing that the primitive sheath or membranous 

 tube of the nerve-fibre corresponds to the sarcolemma of muscle, have pro- 

 posed to designate it as the neurilemma, and to use the term perineurium 

 for the coarser sheathing of the nerves and nervous cords, to which the term 



