TRANSACTIONS OF SECTION I. ou 
Let us consider this for a moment. ‘Take the primary retinal reflex, which 
moves the eye so as to bring the fovea to the situation of the stimulating image. 
From all the receptors in each lateral retinal half rise reflex arcs with a final 
common path in the nerve of the opposite rectus lateralis. Suppose simultaneous 
stimulation of two of these retinal points, one nearer to, one farther from, the 
fovea. If the ares of both points pour their impulses into the final common path 
together, the effect must be a resultant of the two discharges. If these sum, the 
shortening of the muscle will be too great and the fovea swing too far for either 
point. If the resultant be a compromise between the two individual effects, the fovea 
will come to lie between the two points of stimulation. In both cases the result 
obtained would be useless for the purposes of either. Were there to occur at the 
final common path summation of the impulses received from two unlike receptors, 
there would result in the effector organ an action useless for the purposes of either. 
When two stimuli are applied simultaneously which would evoke reflex actions 
that employ the same final common path in different ways, in my experience one 
reflex appears without the other. The result is this reflex or that reflex, but 
not the two together. Excitation of the afferent root of the eighth or seventh 
cervical nerve of the monkey evokes reflexly in the same individual animal, some- 
times flexion at elbow, sometimes extension. If the excitation be preceded by 
excitation of the first thoracic root, the result is almost always extension; if 
preceded by excitation of the sixth cervical root, it is almost always flexion. Yet 
although the same root may thus be made to evoke reflex action of the flexors or 
of the extensors, I have never seen it evoke contraction in both flexors and exten- 
sors in the same reflex response. Of the two reflexes on extensors and flexors 
respectively, either the one or the other results, but not the two together. 
Good opportunity for study of this correlation between reflexes is given in 
the ‘scratch reflex.’ When the spinal cord has been transected in the neck, 
this reflex in a few months becomes prominent. Stimuli applied within a large 
saddle-shaped field of skin (fig. 1 A) excite a scratching movement of the leg. 
The movement is rhythmic flexion at hip, knee, and ankle. It has a frequency 
of about four per second. The stimuli provocative of it are mechanical, such as 
rubbing the skin, or pulling lightly on a hair. The nerve-endings which generate 
the reflex lie in the surface layer of the skin, about the roots of the hairs. A con- 
venient way of exciting these is by feeble faradisation. A broad diffuse electrode 
is applied to some indifferent part of the surface elsewhere, and a stigmatic pole is 
brought to some point in the saddle-shaped area of dorsal skin. This pole is 
formed by, a minute needle with fine wire attached; it is set lightly, so that its 
point just lies among the hair-bulbs. 
Prominent among the muscles active in this reflex are the flexors of the hip. 
If we record their rhythmic contraction we obtain tracings as in figs. 2,3,4. A 
series of brief contractions succeed one another at a certain rate, whose frequency 
is independent of that of the stimulation. The contractions are presumably 
brief tetani. The stimulus to the hair-bulbs of the shoulder throws into action a 
lumbar spinal centre, innervating the hip-flexor much as the bulbar respiratory 
centre drives the spinal phrenicus centre. In the case of the respiratory muscle the 
frequency of the rhythm is, however, much less. 
This reflex is unilateral: stimulation of the left shoulder evokes scratching by 
the left leg, not by the right. Search in the spinal cord for the path of the reflex 
demonstrates that a lesion breaking through one lateral half of the cord anywhere 
between shoulder and leg abolishes the ability of the skin of that shoulder to 
excite the scratch reflex, but leaves intact the reflex of the opposite shoulder. 
In the lateral half of the spinal cord which the reflex path descends, severance 
of the dorsal column does not interfere with the reflex; nor does severance of the 
ventral and the dorsal columns together of that side; no more does severance of 
the grey matter in addition. But severance of the lateral part of the lateral 
column itself permanently abolishes the conduction of the reflex; and it does so 
even if all the other parts of the cord remain intact. The path of the 
reflex therefore descends the lateral part of the lateral column. I enter 
into these details because they help toward the construction of the reflex 
