The symptoms are always worse during hot, damp weather, and when the animals are exposed to the full glare of the sun. The milder cases can be worked without great inconvenience in winter, while they become utterly useless in summer.
Active exertion and increased rapidity of the circulation has a similarly injurious effect. After a period of rest nothing amiss may be noticed, while after a period of work in the sunshine the symptoms become well marked and the difficulty of backing pronounced. Plethora or full feeding aggravates, while spare, laxative diet, laxatives, rest or bleeding relieves.
The affection may become complicated by more active inflammatory action leading to paroxysms resembling those of meningo-encephalitis;—pushing against the wall, rearing up with the fore feet in the manger, acting as if walking or trotting, etc. In other cases the paroxysms resemble those of vertigo; the animal plunging forward, starting to one side, or rearing up and falling back.
Lesions. The pathological anatomy of this disease is that of chronic hydrocephalus. Renault records cases in which the subarachnoid and arachnoid fluids were under the normal while the fluid in the ventricles was increased to a marked extent. This accumulation is often so great that the whole of the surrounding nervous matter is greatly attenuated, the convolutions of the cerebral hemispheres are flattened so that the sulci are all but effaced, the water may shine through at points and even bulge after the manner of a hernia, the ganglia in the ventricle (corpus striatum, optic thalamus, hippocampus) are flattened and atrophied, the base of the cerebrum is thinned and bulges downward, and the olfactory lobes may have their internal cavity greatly distended so that they look like little bladders of fluid. The ependyma may have lost its normal thinness and translucency, having become thick and opaque, and sometimes its surface is granular and rough. The choroid plexus is congested and swollen with infiltration. The brain tissue adjacent is firmly adherent and there is a hyperplasia of its connective tissue constituting a veritable sclerosis. At some points, however, the compressed nervous tissue has undergone degeneration and softening. As might be expected from the pressure of the liquid, anæmia of the brain tissue is a marked feature of the morbid condition.
Other conditions have at times been found in chronic hydrocephalus.
Renault found two long tumors each as large as a hen’s egg projecting from the dura mater into the cerebral hemisphere. In other cases there have been fibrous thickening of the dura mater, exudations on the pia mater, and false membranes on the arachnoid (Röll). Chabert and more recent writers have observed cysts and tumors of the choroid plexus in such cases, but these have been met with not unfrequently in the entire absence of the characteristic symptoms of this disease.
Nature. The affection before us is evidently one in which the majority of the higher brain functions are profoundly depressed or debilitated, and this is accounted for by the accumulating intraventricular liquid pressing on the ganglionic centres in the cerebral hemispheres, and in the floor of the lateral and third ventricles.
Experimentally an approximate loss of sensation, intelligence, spontaneity, will, and muscular power is produced in birds or mammals deprived of their cerebral hemispheres. Colin’s heifer, which had been thus mutilated, would lie in torpor, and though it could be made to get up and walk, it struck its head heedlessly against the wall, and retained in its mouth unchewed, the food that had been placed there. He says of such cases: “they live a long time, move automatically, respire, digest, but they lose, with the sensations, memory, judgment, will, and the most vital instincts of their kind.”
In the dropsy of the ventricles the attenuation and atrophy of the cerebral convolutions produce symptoms which approximate closely to those resulting from their experimental ablation, so that one may fairly attribute the general symptoms in the two cases to the loss of their function. Many of the attendant symptoms, and especially the aberration of smell, sight, hearing and taste, may be referred to the concomitant injuries of the basal ganglia of the brain.
We need not seek in one general answer to resolve the question whether the dropsy or inflammation is the initial lesion. For our present purpose it must suffice, that dropsy with anæmia and atrophy of the cerebral convolutions and basal ganglia produce the symptoms of immobility.