246 IV AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



dogs, and probably the arrangement of these fibres in man is similar. The 

 observations are particularly significant as giving another anatomical basis 

 for the control of the movements in both halves of the body from each 

 cerebral hemisphere. 



The continuous degeneration, coupled with the histological evidence for 

 the absence of intervening nerve-cells, indicates that the cell-bodies in the 

 cortex have axones that extend all the way to the cell-groups of the spinal 

 cord, even as tar as the sacral region. The usual picture of the final connec- 

 tions of the pyramidal fibres shows the collaterals as coming into contact 

 with the large cells in the ventral horns of the cord. On the ground of 

 recent experiments on monkeys and cats, Schafer 1 denies the existence of 

 such ;i din <t association of the two sets of elements. He finds that a large 

 mass of collaterals which degenerate when the pyramidal tract is interrupted 

 end about the large cells in the column of Clarke. 



Returning to consider the arrangement of these cells in the cortex, Ave find 

 that the axones of one group of these cortical cells pass to the cell-groups in 

 the cervical enlargement, while those from others pass to the groups in the 

 lumbar enlargement. It thus happens that if the spinal cord be cut across 

 in the middle of the thoracic region, and then the leg area (see Fig. 78) be 

 stimulated, an electrometer applied to the cut end of the cord will show the 

 passage of nerve-impulses, because the electrometer is applied to a tract of 

 fibres on their way to the lumbar enlargement, and the fibres originate from 

 cortical cells within the region stimulated. 



When, however, the cortical stimulus is made in the arm-area, the electrom- 

 eter being applied as before, no electric change occurs, for the axones of 

 the cells in the arm terminate in the part of the cord containing the cell- 

 groups which control the muscles of the arm, and these groups all lie cephalad 

 to the point of section of the cord. It is evident, therefore, that the arrange- 

 ment is a comparatively simple one — namely, an extension of the axones of 

 the several groups of cortical cells from the different areas for the leg, arm, 

 face, etc., to the axial cell-groups which control the muscles of these parts, 

 and which are situated in the cord. 



The cortical cells in the motor region belong to the group of central cells 

 — /*. e., their axones never leave the central system — and hence they are en- 

 gaged in distributing impulses within it. To the axial cell-groups in the cord 

 they bring impulses, and therefore, from the standpoint of these latter, may 

 be considered as afferent, whereas, owing to the fact that they carry impulses 

 away from the cortex, they are sometimes called efferent. Just how these 

 two sets, the cortical and the cord elements, are numerically related still 

 requires to be worked out. According to one estimate, there arc for the arm, 

 trunk, ami leg, in man. Ti'.lll pyramidal fibres in each half of the cord, or 

 158,222 in the entire cord. 2 The number of fibres in the pyramidal tracts 

 indicates that there certainly is not one fibre for each cell in the axial cell- 



1 Journal of Physiology, vols, xxiii. and x.xiv ; Proceedings of the Physiological Society. 

 » Blocq el ' >zanoff: Gaz. da Hdpil., 1892. 



