THE NERVOUS SYSTEM 297 



is distributed in the albumino-gelatinous substance of the nerve 

 tissue, for this substance is a natural conductor of it and is adapted 

 for holding it and letting it move freely about ; the fluid is kept in 

 by the aponeurotic sheaths which invest this nervous tissue, since 

 these sheaths do not permit of the passage of the fluid. 



Thereafter, when the nervous fluid is distributed throughout that 

 medullary substance which was originally arranged in separate ganglia 

 and afterwards in a cord, its movements probably thrust out portions 

 which become elongated into threads and it is these threads which 

 constitute the nerves. It is known that they spring from their centre 

 of communication, and issue in pairs either from a ganglionic longi- 

 tudinal cord or a spinal cord at the base of the brain, and that they 

 then proceed to their termination in the various parts of the body. 



This no doubt was the method employed by nature for the formation 

 of the nervous system : she started by producing several small masses 

 of medullary substance when the animal organisation had advanced 

 sufficiently to enable her to do so : she then collected them into one 

 chief mass ; through this mass immediately spread the nervous fluid, 

 which had become containable and was kept in by the nervous sheaths : 

 it was then that its movements gave rise to the medullary mass in 

 question, and to the nervous threads and cords which issue from it to 

 the various parts of the body. 



In accordance with this theory, nerves cannot exist in any animal 

 unless there is a medullary mass containing their nucleus or centre of 

 communication ; hence those isolated whitish threads which do not 

 lead to a medullary mass are not to be regarded as nerves. 



I may add to these reflections on the formation of the nervous 

 system, that if the medullary substance has been secreted by the chief 

 fluid of the animal, it is through the agency of the capillary extremities 

 of certain arterial vessels in red-blooded animals ; and since the ex- 

 tremities of these arterial vessels must be accompanied by the ex- 

 tremities of venous vessels, all these vascular extremities, containing 

 coloured blood, are buried in the medullary substance which they have 

 produced, and give rise to the greyish colour, which this medullary 

 substance presents in its external layer : sometimes, indeed, as a result 

 of certain evolutions taking place in the encephalon as it develops, the 

 nutritive organs have penetrated so deeply that the greyish medullary 

 substance is central in some localities, and surrounded in great part 

 by that which is white. 



I may add further, that if the extremities of certain arteries have 

 secreted and then maintained the medullary substance of the nervous 

 system, these same vascular extremities may likewise have deposited 

 the nervous fluid which separates off from the blood and is continually 



