THE SPINAL ANIMAL 31 
periphery will be best dealt with after dealing with that of the motor root-cells themselves, and 
these will now be taken. 
Regarding the efferent root-cells of the spinal cord, these, like the afferent, are divisible into 
three groups — those related to the skeletal musculature, to the skin, and to the viscera. The 
position of the nerve-cell bodies or perikarya of these efferent neurons, unlike that of the afferent, 
is intra-spinal. There is good evidence that they lie embedded in the cord at the same segmental 
level as the point of emergence from the cord of the nerve-fibres they originate.* If into the cord 
a clean incision be made transversely to its length, there ensues degeneration of the motor-root 
fibres immediately at the side of the trauma, and not in front of or behind that level ; the root-fibres 
therefore do not take their origin any distance in front of or behind their point of exit, or they 
would suffer degeneration. This is well seen in regions where each motor root consists of a series 
of rootlets. It proves each rootlet to be a collection of fibres which represents the nerve-cells 
lying in its own particular level of the grey matter, in fact, so to say, which drains only one 
particular cross-level of the cord. This fact can be combined with the further observation that 
each constituent natural rootlet of the motor root contains fibres which, broadly speaking, are 
distributed to all the structures which the entire root innervates.t Each rootlet of the root can 
thus be described as representing in miniature the entire root. It follows that the position of the 
nerve-cells sending motor fibres to any one skeletal muscle is a scattered one, extending throughout 
the whole length of the spinal segments innervating that muscle ; in the limb regions many 
muscles receive their motor fibres from as many as three consecutive spinal roots, and the bodies of 
the nerve-cells innervating those must therefore, inside the cord, extend through the length of 
three whole segments of the cord as a continuous columnar group, and in each transverse level of 
the cord these cells must lie commingled with nerve-cells innervating many other muscles. 
Hence no traumatic injury of the spinal cord can ever paralyze a single muscle alone and apart 
from others. Even the severance of any one whole motor nerve-root cannot paralyze a single 
limb muscle ; the effect of such an injury is to partially impair a large number of the muscles.J 
Analysis of the spinal nerve-supply of the muscles of either limb demonstrates that the 
muscular tissue of the limb is arranged in a number of rays,§ there being one ray for each one 
metamer contributing to the limb. Of these rays the tailmost in the fore-and-aft series are the 
longest ; they extend to the extreme free apex of the limb, whereas the foremost, the most 
rostral, pass only as far as the thigh, the next hindward as far as the knee, the next hindward as 
far as the ankle. In the fore-limb of Macacm rhesus^ the common rhesus monkey, the four hindmost, 
most aboral rays all contribute to the musculature of the hand. When we inquire how these 
units of the segmental architecture of the limbs, these muscular rays, are related to the 
physiological or functional units of the limb musculature, it is at once obvious that the extent and 
boundaries of the two do not coincide. The definitely-bounded, individual and circumscribed 
masses of muscular tissue which arc known as ' the muscles ' of the limb are functional 
elements of its structure as a physiological machine. But each of these functional elements is 
* Sherrington, ' Journ. of Physiol.,' vol. xiv, 1892. A. S. F. Griinbaum, ihid., vol. xvi, 1894. 
t Sherrington, ilnd., vol. xiii, 1892. % Sherrington, ihid. § Sherrington, //'/,/. 
