CHAP. X.] COMMISSURES OF THE BRAIN. 285 



The corpus callosum is a thick stratum of transverse fibres, bent 

 at its anterior and posterior extremities, situate between the hemi- 

 spheres, and forming a floor to a portion of the great median 

 fissure which separates them. Its fibrous structure is very appa- 

 rent to the naked eye, the fibres being collected in coarse bundles. 

 On each side it penetrates into the hemisphere, under cover of the 

 internal convolution already mentioned, which overhangs it in its 

 entire length. It thus connects the anterior, middle, and part of the 

 posterior lobes of each hemisphere; at least, its fibres penetrate the 

 hemisphere at these parts. Foville describes the fibres of this com- 

 missure as being derived partly from the posterior columns of the 

 medulla oblongata, from the optic thalami, from the corpora striata, 

 and, lastly, from the fibrous matter of the hemispheres; and although 

 the demonstration of these numerous sources of origin of these 

 fibres is attended with much difficulty, it nevertheless seems highly 

 probable that the numerous fibres, of which so extensive a stratum 

 is formed, would derive their orgin from several sources. 



The corpus callosum is crossed from before backwards along the 

 median line by two stripes of longitudinal fibres, which, although 

 easily separable, generally lie in close apposition with each other, 

 and form a kind of raphe, dividing the upper surface of the corpus 

 callosum into two equal and symmetrical portions. These fibres 

 seem to be commissural in their office. 



The anterior commissure is a remarkable bundle of transverse 

 fibres, which passes from one hemisphere to the other. It is in its 

 centre a cylinder of fibrous matter, a little thicker than a crow- 

 quill, but becoming very much flattened and expanded at its extre- 

 mities. Its central part is seen at the anterior extremity of the 

 third ventricle, in front of the anterior pillars of the fornix, crossing 

 from side to side, quite free, and unconnected with nervous matter. 

 It plunges on either side into the anterior extremity of the corpus 

 striatum, and, passing through it, its fibres diverge and spread out 

 into the white matter at the floor of the Sylvian fissure, and near 

 the anterior perforated space. 



The posterior commissure crosses the posterior extremity of the 

 third ventricle, and passes transversely between the optic thalami. 

 It is a slender cylinder of fibrous matter, which lies immediately 

 above the anterior orifice of the aqueduct of Sylvius. On each side 

 it seems to sink into the posterior part of the optic thalamus. The 

 base of the pineal body rests upon it, and is connected with it by 

 fibrous matter, which is continuous with the peduncles. 



The soft commissure is a soft pale-gray layer consisting of 



