Nature of phenomenon, arhythmic, hyperexcitability of nerve centres or skin. Nerve section. Head jerking: horse, hard bit, severe check, internal pain, exertion, hypersensitiveness. Treatment: nerve section. Tongue lolling, etc. Flapping of lower lip. Nasal rhythmic movements. Weaving, from impatience, rhythmic. Rocking on hind limbs. Resting foot on coronet. Pawing. Treatment: eliminate irritation, nerve tonics, sedatives, hygienic measures.
This has been observed especially in the dog and may be easily confounded with chorea. The muscles on one side of the face, are twitched at more or less regular intervals, or in other cases there are sudden opening and closing of the lower jaw. The affection has not been satisfactorily connected with any special brain lesion, though as in chorea proper and epilepsy, we must invoke a special disorder or hyperexcitability of the nerve centres presiding over the affected muscles. The clonic spasm may in some cases be due only to a motor impulse from such excited nerve centre, while in others it may be traced backward along the afferent nerves to an oversensitive part of the skin or other organ. In these last purely reflex forms of the trouble it may be possible to correct it by section of the sensory nerves involved.
Convulsive movements of the head as a whole constitute a frequent form of chorea in the dog. It is especially common in horses and shows itself in different forms. A horse with a tender mouth, or which has been used with a hard bit, or with a heavy hand on the reins, or which has been driven with a check rein so short as to be unsuited to its conformation, is liable to indulge in annoying elevation and depression of the head when under the saddle or in harness. The same phenomenon may be shown in connection with violent internal pains, as in strangulated hernia, intussusception, or twisting of the bowels. The habit once formed is not easily corrected, so that careful treatment with the view of prevention is especially to be given.
Another more objectionable, dangerous, and less voluntary motion is the sudden jerking of the head upward, or to one side when excited under the saddle or in harness. The disorderly movements are not, as a rule, seen while the animal is at rest, but seem to be produced under the stimulus of exertion. They appear to be quite involuntary, and suggest the dread caused by the settling or buzzing of an insect about the nose or ears, but occur in the depth of winter in the absence of insect life, as well as in midsummer. The suddenness and involuntary nature of the movement is suggestive of epilepsy, but there is no indication of attendant unconsciousness. From choreic movements it is apparently distinguished, by its presence only when ridden or driven. It is unquestionably associated with hypersensitiveness of the nerve centres, and yet in many cases it appears to be a reflex originating in a specially tender or sensitive part of the skin or mucous membrane. In more than one instance in this college clinic the trouble was corrected by the section of both facial branches of the 5th cranial nerves as they emerged from the infra-orbital foramina.
Some horses double the tongue downward, others upward of the bit; others protrude the tongue and give it a sinuous, serpentine motion which causes alternate protrusion and retraction.
This habit of rapid opening and closing of the lower lip so as to produce a disagreeable flapping noise by striking it against the upper, is seen in many horses and proves a most objectionable trait in harness or saddle animals.
Certain horses apply the protractile end of the nose against the lower lip and spend hours in succession in moving it rhythmically forward and backward, or from side to side.
This consists in a lateral rocking of the head and neck, and sometimes of the chest as well with alternate stepping on the right and left fore feet. It has been supposed to represent the movement of the weaver in working a hand loom, or still better the movement of a caged wild beast in constant turning toward the right and left of the front of his cage. The motions are as regular as a pendulum, and involve the contraction of corresponding groups of muscles on the two sides of the body.
They seem, in some cases, to begin in impatience in waiting for the feed, while other horses in the same row are being attended to first, but when the habit has been formed it may be continued most of the time in the intervals between feeds as well. Nervous horses and those that are hearty feeders are the most subject to this infirmity.
Some horses have a habit of continuously raising one hind limb, others raise the right and left alternately, rocking the hind quarters from side to side, others stand with the heel of one hind foot resting on the front of the coronet of the other, while still others paw continuously with the fore feet while standing in the stall.
Treatment. These various conditions even when begun as an expression of impatience, soon become fixed habits, that prove in the end virtually uncontrollable by an animal, which has no strong will and no consciousness of anything to be gained by resisting the impulse. They become virtual psychoses. In cases in which the habit can be traced to a peripheral irritation, the cutting off of this by complete section of the afferent nerves leading to the irritable nerve centre will sometimes succeed in effecting a cure. In other cases in which the source of the disorder is probably largely central in the cerebral ganglia, nerve tonics, and sedatives, and generally corroborative treatment are the most obvious means of palliation. Such measures are, however, rarely successful. Nourishing food and invigorating outdoor exercise are useful auxiliaries.