Characteristic of reflex postural contraction is its continuance for long spells at a time without obvious fatigue. Attempts are uniformly unsuccessful to reproduce this feature by electrical stimulation of afferent nerves or of the muscle's motor nerve. The suggestion has therefore been made that the postural con traction of muscle is due to a different process in the muscle-fibre from that of the ordinary contraction executing movements and so easily and often studied under electrical stimulation in the laboratory. Forbes, however, has offered the satisfactory explan ation that the postural contraction while the same process as that of ordinary reflex and other contraction involves usually rel atively few motor units at a time and these as they fatigue, are replaced by others. The stretch which is the essential stimulus is automatically shifted to fresh motor-units so soon as those already in operation begin to yield and give out. Grafted, as it were, upon the reflex maintenance of the erect-posture are reflexes of locomo tion which are also certainly spinal in essence, and essentially proprioceptive. The hind-limbs of the dog, after severance of the cord in the thoracic region, begin to step when released from the ground. Their stepping will go on sometimes quicker and some times slower for long periods at a time. Kinematograph analysis of this spinal stepping shows that it is identical with normal stepping except for a slight flexional yield when the limb is vertically on the ground in the normal step. Passively supported mid-flexion of knee and hip even of one hind limb alone at once causes the spinal reflex stepping to cease, in both limbs usually. The proprioceptive stimulus of the spinal stepping may therefore be the passive stretching of the hip and knee-flexor. From obser vations on stepping under narcosis Graham Brown regards the essential factor in "spinal" stepping to be the intrinsic activity of a "spinal stepping" centre or "centres." Certainly it is not un common for an extensor muscle of the limb in isolated attach ment to the myograph and after its own afferent nerve fibres have been wholly destroyed and after all the other muscles of the limb have been put out of play by excision of nerve-section to start stepping on its own accord in the bulbospinal preparation. Skin stimuli, for instance, the contact of the sole with the ground, are not necessary for the reflex stepping. On the other hand the ground pressure against the toe pads and plantar cushion seems an important factor in the gallop—indeed the sudden brief though strong and almost simultaneous extension of all f our limbs in the gallop can in the spinal animal be evoked by pressure simulating the ground pressure, and when applied even to one hind foot only.
The eyeballs are eminently organs which, the gaze being a posture, employ active postures throughout the waking day. The proprioception of the neck-muscles excites from the cervical spinal cord reflex influences acting on the nerves of the eyeball muscles. An instance is : turning of the neck toward one side excites turning of the eyeballs toward the opposite side. Or again, a clockwise partial rotation of the neck round its long axis excites from the neck proprioceptor a compensatory anti-clock wise rotation of the eyeballs. These reflex postures of the eye balls ,are sometimes called compensatory, because they tend to keep truly vertical the vertical of the retina despite displacements of the head by the movements of the neck.
It may be mentioned here that the spinal and the bulbospinal animal (cat) although its composite stretch-reflex enables it when placed in the erect posture to maintain that attitude, even for hours at a time, is yet not capable of assuming that posture when laid, for instance, on its side. But when the mid-brain is re
tained in addition to hind-brain and cord the animal has that power of righting itself. These righting reflexes are likewise postural and largely evoked by the otolith organs. Although not truly spinal reflexes their field of nervous operation overruns into the spinal cord.
In the dog and cat extension of the ipsilateral hind limb can, however, be excited by stimulation of the skin in three limited regions. One of these is the sole of the foot ; smooth pressure be tween the pads excites a strong brief extension. This is called the extensor thrust. It is accompanied by a similar sudden brief ex tension of all three other limbs. This reflex may be related to the action of galloping, and the pressure which excites resembles that which the weight of the body bears on the pads against the ground.
The two other regions are the skin of the front of the groin supplied by the crural branch of the genito-crural nerve, and the skin just below and mesial to the buttock. These always excite the extensor muscles, not the flexors. They may be concerned with sexual acts.