420 THE NERVOUS SYSTEM. 



processes and no distinctly marked axis cylinder prolongation. Their 

 terminal process is very slender and without lateral branches ; while 

 their basal processes extend mainly in a horizontal direction, and some- 

 times communicate with those of adjacent cells. These observations 

 have been confirmed in many particulars by Tuke,* Lewis,f and Char- 

 cot, J and are generally accepted by cerebral anatomists. 



Course of Fibres in the White Substance of the Hemispheres. The 

 white substance of the hemispheres consists mainly of nerve fibres or 

 fibrous tracts belonging to three different orders, namely : 1st. Com- 

 missural fibres ; 2d. Fibres of association ; and 3d. Medullary fibres. 



I. The commissural fibres are those which connect with each other 

 similar parts of the right and left hemispheres. Their principal mass 

 is in the "corpus callosum," or great transverse commissure of the 

 cerebrum, which forms a broad band of white substance at the bottom 

 of the longitudinal fissure and from which the constituent fibres spread 

 out on each side to all the convolutions of the frontal and occipital 

 lobes, and to the upper and posterior portions of the temporal lobe. 

 Next in importance is the " anterior commissure," a narrow cylindrical 

 band of white substance crossing the median line near the base of the 

 brain, a little in front of the optic thalami, and whose fibres radiate on 

 each side to the lower and anterior parts of the temporal lobe. This 

 is accordingly a special transverse commissure for the convolutions 

 situated below the fissure of Sylvius, while the corpus callosum is a 

 general transverse commissure for those situated above, in front and 

 behind it. By these commissural fibres the convolutions of each region 

 of the hemisphere are placed in connection with those of the corre- 

 sponding region on the opposite side. 



II. The fibres of association form tracts lying immediately beneath 

 the gray substance running in a general longitudinal direction, and 

 connecting different convolutions on the same side. Many of them 

 have a short course, connecting the gray substance of adjacent convo- 

 lutions ; others are longer, passing beneath one, two, or even, three 

 intermediate convolutions ; while others run % very extended course, 

 as from the point of the frontal lobe, along the edge of the longitudinal 

 fissure to the end of the occipital lobe, or following the borders of the 

 fissure of Sylvius to the end of the temporal lobe. According to 

 Huguenin, it must be admitted that, in general, all the principal con- 

 volutions of a cerebral hemisphere are connected with each other by 

 fibres of association, in longer or shorter tracts. 



III. The medullary fibres are those which connect the hemispheres 

 with the medulla oblongata. They come up from the spinal cord, 

 through the medulla oblongata, and emerge from the superior border 

 of the tuber annulare, as the crura cerebri. The crura cerebri are 

 divided, about the middle of their thickness by a thin blackish gray 



* Edinburgh Medical Journal, 1875, vol. xx., p. 394. 



f Brain. London, 1878, p. 79. 



J Lepons sur les Localisations dans les Maladies du Cerveau. Paris, 1878, p. 34. 



