The final efferent-root neurone forms the instrument for many different reflex arcs and acts. It is responsive to them in various rhythms and in various grades of intensity. In accordance with this, it seems from experimental evidence to be relatively inde fatigable.
3. In the transition from one reflex to another a final common path changes hands and passes from one master to another. A fresh set of afferent arcs becomes dominant on the supersession of one reflex by the next. Of all the conditions determining which one of competing reflexes shall for the time being reign over a final common path, the intensity of reaction of the afferent arc itself relatively to that of its rivals is probably the most powerful. An afferent arc that strongly stimulates is caeteris paribus more likely to capture the common path than is one excited feebly. A stimulus can only establish its reflex and inhibit an opposed one if it have intensity. This explains why, in order to produce examples of spinal inhibition, recourse has so frequently been made in past times to strong stimuli. A strong stimulus will inhibit a reflex in progress although a weak one will fail. Thus in inhibition of mic turition in the "spinal" dog a forcible squeeze of the tail will do it, but not a weak squeeze. So, likewise, any condition which raises the excitability and responsiveness of a nervous arc will give it power to inhibit other reflexes, just as it would if it were excited by a strong stimulus.
Crossed reflexes are usually less easy to provoke, less reliable of obtainment, and less intense than are direct reflexes. Conse quently we find crossed reflexes usually more easily inhibited and replaced by direct reflexes than are these latter by those former. Thus the crossed stepping-reflex is easily replaced by the scratch reflex, though its stimulus be continued all the time, and though the scratch-reflex itself is not a very potent reflex. But the re verse can occur with suitably adjusted intensity of stimuli.
Again, the flexion-reflex of the dog's leg is, when fully developed, accompanied by extension in the opposite leg. This crossed ex tensor movement, though often very vigorous, may be considered as an accessory and weaker part of the whole reflex, of which the prominent part is flexion of the homonymous limb. When the
flexion-reflex is elicitable poorly, as, for instance, in spinal shock or under fatigue or weak excitation, the crossed extension does not appear. But, where the flexion-reflex is well developed, if not merely one but both feet be stimulated simultaneously with stimuli of fairly equal intensity, steady flexion at knee, hip and ankle re sults in both limbs, and extension occurs in neither limb. The contralateral part of each reflex is inhibited by the homolateral flexion of each reflex. In other words, the more intense part of each reflex obtains possession of the final common paths at the expense of the less intense portion of the reflex. But if the in tensity of the stimuli applied to the right and left feet be not closely enough balanced, the crossed extension of the reflex ex cited by the stronger stimulus is found to exclude even the homon ymous flexion that the weaker stimulus should and would other wise evoke from the leg to which it is applied.
It was pointed out above that in a number of cases the trans ference of control of the final common path FC from one afferent arc to another is reversible. The direction of the transference can caeteris paribus be easily governed by making the stimulation of this receptor or that receptor the more intense. A factor largely determining whether a reflex succeed another or not is therefore intensity of stimulus.
4. A fourth main determinant for the issue of the conflict be tween rival reflexes seems the functional species of the reflexes. Reflexes initiated from a species of receptor apparatus that may be termed noci-ceptive appear to dominate particularly the ma jority of the final common paths issuing from the spinal cord. In the simpler sensations we experience from various kinds of stimuli applied to our skin there can be distinguished those of touch, of cold, of warmth and of pain. The pain ending may be regarded as adapted to a whole group of excitants, a group of excitants which has in relation to the organism one feature common to all its com ponents, namely, a nocuous character.