EXAMINATION OF SOME SPINAL NERVES 143 
connection between the endings of the afferent fibres and motor-cells or other segments may be 
as close as between them and any of the neurons. It follows that, in many instances at least, 
excitation of their own segment applied in minimal or approximately minimal degree to a single 
afferent root, or of a fraction of a single afferent root, evokes pluri-segmental 'motor' discharge. 
Now I have shown that the group of motor neurons innervating a limb muscle, e.g.^ flexor brevis, 
hallucis, or pollicis, is composed of individual neurons, belonging not merely to one but to two 
and three adjacent spinal segments.* From the above it is clear that the rule of segmental 
proximity does not operate to the extent of making motor neurons belonging to one segment in 
the pluri-segmental motor nucleus greatly more accessible by its own afferent root than are, by that 
root, the motor neurons of the other segments composing the nucleus ; in fact they are only very 
slightly if in any degree more so. Here, again, facts lend no countenance to the assertion that the 
collection of fibres in each motor root represents one highly co-ordinate functional synergy. On 
the contrary, they indicate that not the whole motor root, but particular fractional combinations of 
several motor roots are, in spinal reflex actions, to be considered units. The afferent channels of 
the cord treat the pluri-segmental motor stations or nuclei of these limb muscles as entities of 
homogenous structure — as, in fact, physiological units. 
From the foregoing it follows naturally that the reflex centrifugal discharge of the spinal 
cord is pluri-segmental. The rule may be stated thus : in response to excitation even approximately 
minimal of a single afferent root, or even a single filament of a single afferent root, the spinal discharge 
of centrifugal impulses evoked, tends to occur via more than one efferent root, i.e., is pluri-segmental. 
It is interesting to note that this is more strongly the case in the limbs than in the thoracic region 
where the segments are less commingled. The segmental arrangement of the motor cells of the 
spinal cord is said by ScHWALBEt to be more obvious in the tlioracic than in the limb regions. 
But in the limb region, the arrangement, if existent, consists of cell groups quite confluent at the 
boundaries of segments. I look upon the solidarity of the limb as structurally expressed by the 
pluri-segmental character of the motor nuclei. Kaiser'sJ measurements lend no colour to 
Schwalbe's assertion of a segmental grouping of the motor cells. With care, the reflex discharge 
can, as above stated, be confined in the intercostal series to one segmental region, the spinal 
centrifugal discharge being then uni-segmental. This circumstance is in accord with the uni- 
segmental innervation of these muscles as compared with the pluri-segmental innervation of most 
limb muscles. 
This serves to emphasize what I have frequently insisted on, namely, the physiological 
homogeneity of limb muscle and nerve-trunk, and the physiological heterogeneity, in spite of 
morphological unity, of the spinal nerve-root in the limb region ; the spinal nerve-roots of the 
thoracic region are far less heterogenous. The peripheral nerve-trunk is the physiological collection 
of nerve-fibres, e.g., flexors collected together, vaso-dilators included with motors to muscles, &c. 
The nerve-root is the morphological collection ; it contains, commingled into one, such hetero- 
* Sherrington, 'Joiirn. of Physiol.,' vol. 13, p. 621, 1892. 
f Schwalbe, 'Lehrbuch Her Neurol.,' 1881 ; also Luderitz, 'Arch, tur Anat. und Physiol.,' 1881. 
\ ' Ganglienzellen des Halsmarlces,' Haag, 1891. 
