BRAIN. 61 



with the spinal cord; that is to say, those which are usually traced upwards 

 from the columns of the spinal cord, through the medulla oblongata iuto the 

 cerebrum, chiefly by means of the anterior pyramids, fasciculi teretes, and 

 restiform bodies, passing through the pons and crura cerebri, to expand into 

 the corpora striata, optic thalami and convolutions (corona radiata), and by 

 means of the restiform bodies, into the cerebellum. 



The other two classes of white fibres in the brain are commissural ; some of 

 the commissures serving to connect different parts of the same hemisphere 

 together (as the fornix, the processus e cerebello ad testes, &c.), or even different 

 parts of the same section or organ, as the arciform fibres of the medulla. Most 

 of these commissures are longitudinal ; while others as the corpus callosum 

 and the transverse fibres of the pons Varolii are transverse, serving to con- 

 nect opposite hemispheres together, and thus probably securing the single 

 action of a double organ. 



The following is Mr. Lockhart Clarke's account of the intimate structure of 

 the cerebral convolutions : 



" Most of the convolutions, when properly examined, may be seen to consist 

 of at least seven distinct and concentric layers of nervous substance, which are 

 alternately paler and darker from the circumference to the centre. The lami- 

 nated structure is most strongly marked at the extremity of the posterior lobe. 

 In this situation all the nerve-cells are small, but differ considerably in shape, 

 and are much more abundant in some layers than in others. In the superficial 

 layer, which is pale, they are round, oval, fusiform, and angular, but not nume- 

 rous. The second and darker layer is densely crowded with cells of a similar 

 kind, in company with others that are pyriform and pyramidal, and lie with 

 their tapering ends either towards the surface or parallel with it, in connection 

 with fibres which run in corresponding directions. The broader ends of the 

 pyramidal cells give off two, three, four, or more processes, which run partly 

 through the white axis of the convolution, and in part horizontally along the 

 plane of the layer, to be continuous like those at the opposite ends of the cells, 

 with nerve fibres running in different directions. The third layer is of a much 

 paler color. It is crossed, however, at right angles by narrow and elongated 

 groups of small cells and nuclei of the same general appearance as those of the 

 preceding layer. These groups are separated from each other by bundles of 

 fibres, radiating towards the surface from the central Avhite axis of the convolu- 

 tions, and together with them form a beautiful fanlike structure. The fourth 

 layer also contains elongated groups of small cells and nuclei, radiating at right 

 angles to its plane ; but the groups are broader, more regular, and, together 

 with the bundles of fibres between them, present a more distinctly fanlike 

 structure. The fifth layer is again paler and somewhat white. It contains, 

 however, cells and nuclei which have a general resemblance to those of the 

 preceding layers, but they exhibit only a faintly radiating arrangement. The 

 sixth and most internal layer is reddish-gray. It not only abounds in cells like 

 those already described but contains others that are rather larger. It is only 

 here and there that the cells are collected into elongated groups, which give 

 the appearance of radiations. On its under side it gradually blends with the 

 central white axis of the convolution, into which its cells are scattered for some 

 distance. 



" The seventh layer is this central white stem or axis of the convolution. On 

 every side it gives off bundles of fibres, which diverge in all directions, and in 

 a ianlike manner towards the surface, through the several gray layers. As they 

 pass between the elongated and radiating groups of cells in the i'nner gray 

 layers, some of them become continuous with the processes of the cells in the 

 same section or plane, but others bend round and run horizontally, both in a 

 transverse and longitudinal direction (in reference to the course of the entire 

 'convolution), and with various degrees of obliquity. While the bundles them- 

 selves are by this means reduced in size, their component fibres become finer 



