BRAIN AND NERVES. 267 



of the brain consists of a very fine granular soft mass, in which 

 here and there larger grains are deposited in nests or layers. 

 The larger grains are free ; the very fine grains, whenever their 

 softness, sraallness, and transparency allow them to be seen, are 

 united together in rows by very delicate threads. The white or 

 medullary substance shows also many distinct fibres, continua- 

 tions of the cortical fibres, and passing in the same direction to- 

 wards the base of the brain. They are not simple cylindrical 

 threads ; but resemble strings of pearls, the pearls not being in 

 contact, but kept at a little distance from each other. They are 

 always straight, commonly parallel, sometimes crossing each 

 other ; in some rare cases they may be seen splitting into two, 

 but not anastomosing. Near the bases of the brain we find be- 

 tween knotty bundles of fibres much thicker fibres always isolat- 

 ed. These last show distinctly an inner and outer limit of their 

 walls, from which it is evident that they are hollow tubes. We 

 may call them varicose tubes or canals, because they swell out 

 in many places, resembling little blown bladders attached to each 

 other by a narrow tube. 



The interior of these varicose tubes is quite transparent, so 

 that we might conceive them to be filled with vapour or with 

 water. The milk-white colour which they have when viewed by 

 the naked eye, is owing to the liquid contained in them, being of 

 a milk-white colour, and somewhat muddy. This matter even 

 when magnified 3000 times, does not exhibit any granular sub- 

 stance as the cause of this muddiness. The milk-white colour 

 is wanting in the cortical substance of the brain. It consists of 

 the points or beginnings of the varicose tubes, which exhibit 

 their walls or boundaries, but want the bulky contents which 

 exist in the medullary tubes. From this it is evident that the 

 white colour is owing entirely to the contents of the tubes. 

 When the tubes are torn, they contract ; but nothing can be per- 

 ceived coming out of them. The large brain tubes converge to- 

 wards -the place in the basis of the brain from which the nerves 

 proceed, and pass over their origins. 



The nerves of the senses seeing, hearing, and smelling, to- 

 gether with the great sympathetic, consist of cylindrical parallel 

 tubes about T i 5 th of a line in diameter, running close to each 

 other, but not anastomosing. They are united in bundles, 

 which again form larger bundles, called nervous cords. Each 



