942 DECUSSATION OF PYRAMIDS. [BOOK in. 



different nature and origin. The fibres which in sections below 

 were seen in the act of crossing are now gathered into masses 

 of longitudinal fibres, the pyramids, (Py.) one on each side of the 

 anterior fissure, each with a sectional area of a rounded triangular 

 form clearly marked out from the surrounding structures; the 

 section is taken above the decussation of the pyramids. Or, 

 tracing the changes from below upwards we may say that the 

 decussation is now complete ; on each side the whole of the 

 crossed pyramidal tract of the spinal cord has, in the region of 

 the bulb below the level of the present sections, crossed over to 

 the other side, and joining with the direct pyramidal tract of 

 the anterior column of the cord of the same side has become the 

 pyramid of the bulb. In other words, the decussation of the 

 pyramids is, as we have already hinted, the passing off from 

 each pyramid, and the crossing over to the opposite side of the 

 cord, of those fibres which are destined to become the crossed 

 pyramidal tract of the spinal cord of the opposite side, while 

 the rest of the pyramid pursues its course on the same side as 

 the direct pyramidal tract. 



606. In the spinal cord the bottom of the anterior fissure is 

 separated from the central canal by nothing more than the 

 anterior white commissure and a narrow band of grey matter, 

 composed of the anterior grey commissure and of part of the 

 central gelatinous substance. During the decussation of the 

 pyramids, the decussating fibres push, as it were, the central 

 canal with its surrounding grey matter to some distance from 

 the bottom of the anterior fissure. In sections above the decus- 

 sation the bottom of the fissure does not again approach the 

 central canal, but continues to be removed to some distance from 

 it, and, as we pass upwards, to an increasing distance, by the 

 interposition of tissue which consists largely of decussating fibres. 

 These however, though they seem to continue on the decussation 

 of the pyramids, are shewn by the embryological and degeneration 

 methods to have no connection with the pyramids, but to belong 

 to another system of decussation. As we have seen ( 565) the 

 anterior commissure along the whole length of the cord contains 

 decussating fibres. Some of these in the upper part of "the cord 

 are fibres crossing from the direct pyramidal tract of one side to 

 the grey matter of the other side, and so may be regarded as part 

 of the whole pyramidal tract ; but others are of different origin ; 

 and even in the region of the actual decussation of the pyramids 

 some of the fibres which cross over do not belong to the pyramidal 

 tract. This system of decussating fibres becomes increasingly pro- 

 minent above the decussation of the pyramids, and through it the 

 ventral area of the bulb between the central canal and the anterior 

 fissure is much increased. The fibres as they cross form a middle 

 line of partition, the raphe (Fig. 109, 4, 5, r), which increases in 

 depth in the upper parts of the bulb, and on each side of the raphe 



