404 NERVES. STRUCTURE. COMMUNICATIONS. 



communicating it to the muscles, that the latter may instantly remove 

 the arteries from impending injury. 



The microscopic examination of a cerebro-spinal nerve shows it to 

 be composed of minute fibres, resembling those of the brain, and con- 

 sisting of a neurilemma enclosing a pellucid fluid during life, and an 

 opaque, white, curd-like substance after death. The chief difference 

 between the fibres of the nerves and the cerebral fibres is the larger 

 size (gsVo to 56 1 06 of an inch) of the former, the greater thickness of 

 their neurilemma, and a consequent indisposition to the formation of 

 varicose enlargements upon compression. The primitive fibres, or fila- 

 ments, are assembled into small bundles and enclosed in a distinct 

 sheath, constituting a.funiculus; the funiculi are collected into larger 

 bundles or fasciculi, and a single fasciculus or a number of fasciculi con- 

 nected by fibro-cellular tissue, and invested by a membranous sheath, 

 constitute a nerve. The funiculi, when freshly exposed, present a 

 peculiar zigzag line across their cylinder, which is most probably pro- 

 duced by a wrinkling of the neurilemma, as it is destroyed by making 

 extension upon the nerve. 



Communications between nerves take place either by means of the 

 funiculi composing a single nerve, or of the fasciculi in a nervous 

 plexus. In these communications there is no fusion of nervous sub- 

 stance, the cord formed by any two funiculi is constantly enlarged, 

 and corresponds accurately with their combined bulk. Microscopic 

 examination substantiates this observation, and shows that the primi- 

 tive fibre passes unchanged from one funiculus to the other, so that 

 the primitive fibre is single and uninterrupted from its connection 

 with the cerebro-spinal axis to its terminal distribution. A nervous 

 plexus consists in a communication between the fasciculi and funi- 

 culi composing the nerves which are associated in their supply of a 

 limb or of a certain region of the body. During this communication 

 there is an interchange of funiculi, and with the funiculi an inter- 

 change of fibres. 



The Sympathetic system consists of numerous ganglia, of communi- 

 cating branches passing between the ganglia, of others passing be- 

 tween the ganglia and the cerebro-spinal axis, and of branches of 

 distribution which are remarkable for their frequent and plexiform 

 communications. The sympathetic nerves also differ from other nerves 

 in their colour, which is of a greyish pearly tint. Examined with 

 the microscope they are seen to be composed of an admixture of grey 

 and white fibres ; the white fibres belong to the cerebro-spinal system : 

 the grey are much smaller than the white, less transparent, and the 

 neurilemma is less easily distinguishable from its contents : some of the 

 nerves are composed of grey fibres only, without any admixture of 

 white. The sympathetic ganglia contain the nerve cells observed in 

 the grey substance of the brain ; they are firmer in structure and en- 

 closed in a strong investing capsule, the latter being made up of 

 nucleated corpuscles. The fasciculi of fibres entering the ganglion 

 become divided and form a plexus around the globules ; they then 



