THE CINERITIOUS AND MEDULLARY NEURTNE. 553 



ganglia and running from them to the different parts of the body. 

 But they are, in fact, merely agents of communication, between 

 the various ganglia of the nervous system, and the different 

 organs of sense and motion ; some fibres carrying impressions 

 from without inwards to the ganglia ; others in the opposite 

 direction, transporting the motive or other influences which these 

 impressions have excited in the ganglia, (especially those of the 

 cerebrum,) outwards to the muscles. 



— It has been already observed, that the medullary substance, 

 consists, when closely analyzed, of minute elementary fila- 

 ments, each one surrounded by a separate neurilema, or cylin- 

 drical sheath, which isolates it in its passage through the dif- 

 ferent parts from its origin to its termination, and at the same 

 time serves to support it in connexion with the coats of sur- 

 rounding filaments, a number of which go to constitute a 

 nervous cord. In the brain and spinal marrow this same fila- 

 mentous arrangement exists, though more difficult of observation 

 there, in consequence of the nervous filaments, as they are 

 extended upwards into the brain, being no longer rolled up in 

 cords, but placed in juxtaposition with each other so as to form a 

 solid mass, each filament at the same time being compressed 

 into a smaller space, so as to be diminished in its diameter. 

 — The diameter of the primitive filaments found in the nerves 

 is, as has been already stated, about ,JS.th part of a line, or 

 nearly the 3500th part of an inch. But as they pass up into 

 the brain through the spinal marrow, or through the encephalic 

 nerves, they become so much lessoned in their diameter as not 

 to measure more in the medullary substance of the brain than 

 the TCoo^h part of an inch, or more than T^goo^h where they 

 terminate in the cortical portion. From this cause and the 

 great delicacy of the investing neurilema there is much diffi- 

 culty in tracing their individual course within the cranium, but 

 the fact that a large portion of the white fibres of the brain are 

 continuous with those of the nerves, will no longer admit of 

 being called in question. We say a part of the white fibres 

 of the brain, for there is some reason to believe, that accord- 

 ing to the doctrines of Gall, some white fibres, arise de novo, 

 in the gangliform masses of cineritious matter placed at the 

 VOL. II. 47 



