592 THE BRAIN. 



so crossing the fibres push aside the bottom of the anterior fissure. When 

 the course of these fibres is investigated, either by simple microscopic obser- 

 vation, or still better by the method of degeneration, it is found that they 

 may be traced from the anterior column of one side, across the anterior 

 commissure, through the neck of the anterior horn to the lateral column of 

 the opposite side, and to that part of the lateral column which we have pre- 

 viously described as the crossed pyramidal tract. 



In a section a little higher up (Fig. 132, 2), these decussating fibres form 

 on each side a large strand which starts from a part of the anterior column, 

 now becoming distinctly marked off as the pyramid (Py.\ an( ^ * s apparently 

 lost in the reticular formation, but in reality passes on to the crossed pyram- 

 idal tract of the lateral column. This strand, as it crosses over, completely 

 cuts off the head of the anterior horn from the more central gray matter, 

 and forms with its fellow a large area of decussating fibres between the 

 bottom of the anterior fissure and the central gray matter. When a surface 

 view of the bulb is examined the decussation is seen to be effected by alter- 

 nate bundles, passing now from right to left, now from left to right ; and in 

 transverse sections we find correspondingly that the anterior fissure appears 

 bent now to the left and now to the right, according as the section .cuts 

 through a bundle passing from left to right or from right to left. 



In sections still higher up (Fig. 132, 3 and 4) this conspicuous strand of 

 fibres crossing obliquely from side to side will be no longer seen ; decussating 

 fibres are seen dorsal to the anterior fissure, but these, of which we shall speak 

 presently, are of different nature and origin. The fibres which in sections 

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

 tudinal 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 upward, 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. 



519. 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 gray matter, composed of the anterior gray 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 gray matter to some distance from the bottom of the anterior 

 fissure. In sections above the decussation the bottom of the fissure does not 

 again approach the central canal, but continues to be removed to some dis- 

 tance from it, and, as we pass upward, 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 shown by the embryological and degeneration methods to have no con- 

 nection with the pyramids, but belong to another system of decussation. As 

 we have seen ( 477) 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 gray 



