114 NERVE TRUNKS. 



fibres ; and that in all cases there exist complete circuits, into 

 the formation of which central nerve cells, peripheral nerve cells, 

 and nerve fibres enter. All these elements are in structural 

 connection with each other " (" Biopl.," p. 174). 



It would be superfluous to go into the details of the contro- 

 versy, as Dr. Beale has reviewed it so recently, including the 

 elaborate memoir of Dr. Klein,* but some points will be 

 touched upon in discussing the theory of muscle and nerve 

 action. 



On the Nerve Trunks. "Every peripheral nerve network is con- 

 nected with its nerve centre by fibres, and whenever the distance 

 between the centre and peripheral organ is considerable, the nerve 

 fibres are protected from each other, and from the tissues through 

 which they pass, by a thick layer of oleo-albuminous matter, 

 which forms an investment to each bundle of delicate fibres, 

 by which it is insulated and separated from its neighbours, 

 and from other structures, by a distance equal to from five to 

 twenty times its own diameter. When the trunk passes 

 through narrow canals, as through holes in the cranium, this 

 insulating protective covering is much reduced in thickness, so 

 that a large bundle of nerve fibres is made to pass through a 

 space not more than one-fourth of the diameter which the 

 nerve-trunk possesses in other parts of its course. The fibres 

 which have this thick covering are known as ' dark-bordered 

 fibres/ from the dark double contour line they always exhibit 

 when examined in water or weak serum ; the covering itself is 

 known as the ' white substance of Schwann,' or the ' medullary 

 sheath'" ("Biopl.," p. 175). 



It forms a tolerably uniform layer round the core, which it 

 probably insulates and protects, like the gutta-percha does the 



opper wire of the marine cable. Also this white substance of 

 Schwann is so constituted as to interfere with the free passage 

 of fluid. It is not very permeable to aqueous or albuminous 

 solutions, and thus a uniform degree of moisture of the axis 



* <: Monthly Microscopical Journal,"^ vol. vii. p. 156. Dr. Beale's 

 reply is at p. 253 of the same volume, and appears lo me satisfactory 

 in all the essential points. 



