Swimming.—The reflex mechanism of swimming has been studied in lower forms by Coghill, Herrick, van Gehuchten and others. In amblystoma tigrinum the mechanism, according to Coghill, consists (t) of a series of afferent neurones whose dendrites supply skin and myotomes (muscles) and whose ax ones ascend the spinal cord to the medulla, forming a common afferent path; (2) of a group of commissural neurones in the medulla which connect the afferent path with the efferent path; and (3) a chain of efferent neurones, forming a common motor path, which supplies all the myotomes of that side with motor fibers. Of course this mechanism is duplicated; an afferent and an efferent path are present on both sides, and each afferent is connected with the opposite efferent path by the commissure in the medulla. This mechanism may be set into operation by any external stimulus. If an extero-ceptive stimulus is applied to the right side of the body, the impulse traverses some per ipheral nerve to the cord and ascends the cord to the medulla; there it passes through the commissure to the opposite side and excites a motor impulse in the efferent neurones, which descends the common motor path of the left side. As the motor impulse proceeds down that side it causes contraction of the myotomes in succession, the contraction wave advancing gradually toward the tail. The result is a bending of the head, and later of the tail, toward the left side, away from the point of stimulation. The contraction of the myotomes constitutes the second stimu lus; it excites proprio-ceptive impulses (muscle-sense impulses) which, first, inhibit the contraction of the myotome in which they originated and, second, ascend the afferent path to the medulla, cross over to the opposite efferent path and excite impulses that cause contraction of the right myotomes in suc cession from head to tail. While left flexion is still present in the tail, the head is flexed to the right; the first S-flexure is the result. The contraction of the right myotomes also glides tail ward and produces proprio-ceptive impulses which inhibit their own contractions and, passing through the afferent, commissural and opposite efferent path, cause contraction of the left myotomes in regular order from before backward. The head is thus bent to the left while the tail is still bent toward the right; the reversed S-flexure is the result. The alternating production of the S-flexures constitutes the swimming movement.
Posture Reflex.—Posture both in standing and walking is maintained by a reflex mechanism whose stimuli originate within the anti-gravity, extensor muscles. This synergic group of muscles is kept in a state of gentle tonic contraction, or reflex tonus, by the proprio-ceptive impulses induced in the neuro muscular and neuro-tendinous spindles of the muscles by their own contraction. Those proprio-ceptive impulses (muscle sense impulses) ascend the afferent nerves and the posterior column of the spinal cord to the nuclei gracilis and cuneatus of the medulla; thence they continue to the cerebellar cortex through the arcuate fibers in the restiform body. In the cere bellar cortex, they excite impulses that secure a gentle, steady contraction of the anti-gravity muscles. The path traveled from the cerebellar cortex to those muscles probably runs through the brachium conjunctivum to red nucleus and thala mus; then descends the rubro-spinal and thalamo-spinal tracts to the gray substance of the cord, whence the motor fibers of the spinal nerves carry the impulses to the muscles which pre serve the erect posture.
A decapitated dog or cat cannot stand; the end-brain may be removed and the erect posture be maintained; but, if the section be made through the thalamus or below it, the animal is unable to stand. Hence, the coordinating impulses from the cerebellar cortex probably reach the spinal cord by way of the red nucleus or thalamus. It is evident that walking is possible only when the posture mechanism and the mechanism of stepping are both in operation.
Stepping reflex step has been studied by Goltz, Loeb, Sherrington, and others. In the dog and cat the mechanism of the step for the hind limbs most closely resembles the stepping mechanism of man. In the decapitated dog this is as follows: (1) The ajerent spinal neurones which supply the muscles, skin, etc., of the hind limbs. (2) Two systems of efferent neurones on each side, one supplying the flexors and the other the extensors of the limb, so adjusted or attuned that stimulation of one system inhibits the action of the other on the same side; and, furthermore, these efferent systems are so adjusted in opposite sides of the cord that stimulation of the flexor neurones of one limb equally activates the extensor neurones of the opposite limb. (3) The reflex arcs are com pleted by intrinsic neurones of the cord, associative and corn missural, which join the afferent and efferent limbs together.
This mechanism may be set into operation by flexing or ex tending one of the limbs, by applying the faradic current, by pinching, etc. Suppose the right hind limb is pinched. An afferent impulse ascends to the cord; it is transferred to the efferent neurones of both sides of the cord; and it excites the flexor system of the same side and the extensor system of the opposite side. The result is, first, withdrawal of the right hind limb from the offending object and, second, extension of the left limb in an effort to run away. The stepping motion is then con tinued by proprio-ceptive impulses (muscle-sense impulses) excited in the neuromuscular and neuro-tendinous spindles by the tension of the contracting muscles. The contraction of the flexors in the right limb is inhibited by a cumulative extensor impulse, excited by the tension of the flexors, which becomes effective when contraction is complete; this impulse excites the extensor neurones of the same side and, passing through the commissure of the cord, stimulates the flexor neurones of the left limb. The proprio-ceptive stimuli excited in the contracting muscles will continue the alternate flexion of one limb and the extension of the other until the stepping is arrested by fatigue or otherwise.
Educated Movements of the acquired move ments by long practice become automatic, they are evidently performed by a combined mechanism of posture and movement similar to that of walking. The spinal mechanism secures the reciprocal stimulation and inhibition of antagonistic systems of neurones, the flexors and extensors; and the cerebellar mech anism, as described under posture reflexes, so coordinates the spinal mechanism as to obtain the proper successive postures of hands and digits.