Much more common are factors which act primarily on the nerve centres. Poisoning with lead, poppy, belladonna (sheep and goats, Gerlach), lolium temulentum (Klüver), tobacco, carbon bisulphide, cryptogams (musty fodder), mushrooms, astragalus mollissimus, etc., abolishes vision more or less perfectly. Common illuminating gas and carbon monoxide have shown this effect on dogs and chickens. Quinia subcutem has induced the condition in dogs (Becker and Eversbusch). More or less complete amaurosis is noticed in connection with heat apoplexy in horses and fat cattle.
Cerebral concussion from blows on the head, knocking the head against a post or wall, or from violent falls induce blindness which is to be attributed rather to lesion of the brain. The same remark applies to cases that appear during the progress of brain disease, parasitic or otherwise, to those that occur in connection with overloading of the stomach or intestines, and from gestation (Riss).
In cases appearing after severe depletion, like profuse hæmorrhage, diarrhœa, or diuresis, or excessive secretion of milk (bitch), the anæmia of both eye and brain may be taken into account.
When amaurosis develops during some inflammatory or infectious disease, as in parturition fever in cows, the immediate cause may sometimes be found in embolism of the retinal, or cerebral arteries.
Symptoms. The most striking feature of amaurosis is the constant excessive dilatation of the pupil. This is the same in light and darkness, and in fully developed cases the animal may be taken from perfect darkness to the full glare of sunlight or electric light without the slightest contraction of the pupil. The pupillary reflex to light is entirely lost. In the slighter cases (amblyopia) there remains some slight response of the iris to light but always far short of the normal. The widely dilated pupil admits a flood of light, and in the absence of cataract, lights up the chamber of the vitreous to an unusual degree. The blue or yellowish reflection of the tapetum is very striking, and led to the obsolete names of gutta serena and glass eyes. Sometimes the disease is unilateral and at others bilateral. In case it is unilateral there is a strong presumption that the causative lesion is in the affected eye or in the optic nerve in front of the chiasma. In the bilateral cases it is more likely to depend on disease of the brain or some more distant organ.
Certain important indications are to be drawn from the expression of the face, the position of the head and ears and the mode of locomotion. In all blind animals the face is without expression. The dilated pupils give no suggestion of mind looking through them, they have on the contrary an uniform stony stare. There is no sense of fire or life, but the face is like a dull sculptured mass. The immobile face finds a sharp contrast in the alertness of the ears, which may be carried one forward and another back, but fixed and yet ready to turn to catch every sound. In the horse the head is usually carried a little to one side for the same reason, and with the nose elevated (star-gazer) and nostrils dilated. The ox inclines to carry the head low, while the dog keeps his nose near the ground to guide himself by the sense of smell. All blind animals are disposed to move slowly and carefully, and the horse acquires a habit of high stepping to avoid stumbling.
A common and important test where both eyes are equally affected and cannot be contrasted with each other, is to strike the lower part of the face with the hand and immediately threaten to repeat the blow. If the animal can see, it will seek to jerk the head out of the way as soon as the hand is raised for the second blow, while if blind there will be no such movement provided the motion of the hand has not been such as to induce a current of air on the face.
Lesions. Blindness, or the general symptoms of amblyopia or amaurosis, may attend on the atrophy of the retinal fibres in connection with inflammatory exudation or blood extravasation on, in, or beneath, the retina. In other cases the lesion is in the papilla which is inflamed and swollen with the woolly aspect characteristic of choked disc. In others there is congestion of the optic nerve behind the bulb, with exudation into its substance or beneath its sheath, or the nerve has already undergone extensive atrophy with thickening of its neurilemma. In other cases there is atrophy of the arteria centralis retinæ, or tumors of the orbit or optic foramen pressing upon the nerve. Cases of this kind are reported in the larger animals by Leblanc and Tscheulin, and in dogs and ducks by Hilbert.
Much more frequently the determining lesions are situated in the brain or its meninges. In sheep it is very common from the development of cœnurus cerebralis over the optic lobes. Kühnert found a cyst with delicate walls in the brain of a horse affected in this way. Amaurosis is occasionally seen in connection with the cerebral abscesses which form in complicated cases of strangles or in pyæmia, also in cases of cholesteatoma of the choroid plexus. Other conditions of its occurrence are hydrocephalus, meningitis, hypertrophy of the pineal gland, fracture of the cranium, tumors of the cranium or dura mater, embolism or aneurism of the cerebral arteries, hypertrophy and induration of the dura mater (ox, Leblanc), or sanguineous apoplexy. In a certain number of cases there may be no lesion of brain or eye, or only a congestion of the former in connection with lead poisoning, rye grass poisoning or other gastric disorder, or of gestation.
Treatment. Success will depend on the sympathetic nature of the condition or on the transient and removable character of its causative factor. When the condition is but a symptom of overloaded stomach or a transient poisoning by vegetable or mineral agents a direct recovery may be expected to follow their disuse and elimination from the system. This may be hastened by the exhibition of laxatives and diuretics, and in the case of lead by sulphuric acid followed by iodide of potassium. In cases of snow blindness it is only necessary as a rule to place the animal in the shade until the over-stimulation shall have subsided. In all these cases the attack has come on abruptly and without any local symptom of ocular hyperæmia and this with the preservation of the general senses can always be looked on as a favorable indication.