NERVES STRUCTURE COMMUNICATIONS. 361 



ever, be received literally ; for each nerve is developed in the pre- 

 cise situation which it occupies in the body, and with the same rela- 

 tions that it possesses in after life. Indeed, we not unfrequently 

 meet with instances in anencephalous foetuses where the nerves are 

 beautifully and completely formed, while the brain and spinal cord 

 are wholly wanting. The word origin must, therefore, be consi- 

 dered as a relic of the darkness of preceding ages, when . the 

 cerebro-spinal axis was looked upon as the tree from which the 

 nerves pushed forth as branches. In their distribution the spinal 

 nerves for the most part follow the course of the arteries, particu- 

 larly in the limbs, where they lie almost constantly to the outer side 

 and superficially to the vessels, as if for the purpose of receiving 

 the first intimation of danger and of communicating it to the' mus- 

 cles, that they 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 

 consisting of a neurilemma enclosing a soft, homogeneous nervous 

 substance. The chief difference between the fibres of the nerves 

 and the cerebral fibres is a somewhat greater opacity and more 

 granular appearance of the contents of the minute cylinders of the 

 former; a greater thickness of their neurilemma, and an indisposition 

 to the formation of varicose enlargements upon compression. The 

 neurilemma presents the same two layers which exist in the cere- 

 bral fibres. The primitive fibres, or filaments, are assembled into 

 small bundles and enclosed in a distinct sheath, constituting afuni- 

 culus; the funiculi are collected into larger bundles or fasciculi, and 

 a single fasciculus or a number of fasciculi connected by 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 produced by the 

 arrangement of the primitive fibres, or possibly by some condition 

 of the neurilemma. This appearance is destroyed by making exten- 

 sion 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 pri- 

 mitive fibre passes unchanged from one funiculus to the other, so 

 that the primitive fibre is single and uninterrupted from its connec- 

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

 nervous plexus consists in a communication between the fasciculi 

 and funiculi 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 interchange of fibres. 



The Sympathetic system consists of numerous ganglia, of commu- 



46 



