120 Sketch of a General Theory of the {Aue. 
merely inaccurate, but altogether false ; for this would prove, that 
both roots were at once nerves of sensation and of volition: but, 
not being true, the case is certainly somewhat altered. Unluckily 
for Dr. Leach, it is his own statement which is inaccurate. In his 
“ careful examination of the structure of the spinal mass of nerves,” 
the Doctor has absolutely mistaken the branches for the roots of 
these nerves! It is from the branches that the nerves he alludes to 
go off; for, however lucky this may be for humanity, since it pre- 
vents our moving with only one half the body, and feeling only 
with the other, it is certainly unfortunate for the Doctor’s argument 
that neither to skin nor muscles is the slightest twig given from the 
yoots. These roots then. combine, communicate, and even cross by 
twigs, in order to form a trunk ; and, that the Doctor may not be 
put to the trouble of avother “ careful examination,” if he will 
only cross the fingers of one of his hands between those of the 
other, he will have a tolerable conception of the trunk so formed, 
remembering, however, that only about half the fibrils of either 
root do so cross, while the other half, instead of crossing to the 
opposite branch, runs onward in the branch of the same side. A 
rather greater number of fibrils, indeed, pass from the posterior 
root to the anterior branch than from the anterior root to the poste- 
rior branch, because the anterior branch, being destined to supply a 
greater portion of the body, requires to be larger. I do not find 
this decussation described in any anatomical book, which I have at 
hand ; but the slightest inspection will demonstrate it. The law 
of this decussation is maintained even in very inferior animals ; for, 
in those which have no vertebrae and in which the spinal marrow is 
formed below the cesophagus by the union of the two crura of the 
cerebellum, though the two fasciculi generally remain distinct 
throughout the greater part of their length, yet they always unite 
at different spaces by knots whenever a nerve is given off! Thus 
each branch is composed from both roots: and it is only from the 
branches thus composed, and by no means from the roots, that the 
nerves the Doctor speaks of are distributed: hence it is not won- 
derful that they give loth sensation and voluntary motion. These 
branches, however, the Doctor alls “ the two roots of nerves of — 
each half* of the spinal marrow, namely, the anterior and poste- 
rior ;” and asserts, as is seen above, that these identical roots of 
each half of the spinal marrow “ go to different parts of the body!” 
Every anatomist and every anatomical work declares that from the 
roots no twig ‘proceeds either to skin or muscles; and if it were not 
obvious that the Doctor had mistaken the branches for the roots, I 
should be apt to think that, in ‘his “ careful examination of the 
structure of the spinal mass of nerves,” the Doctor had refuted the 
whole of them. . 
I have now to mention, that even some of those anatomists who 
* Thus, too, the Doctor after all allows that there are a sternal and dorsal half 
of thespinal marrow, ~ ' 89, ony Aen ee a a 
