Friedberger and Fröhner found apoplexy quite frequent in sheep, ox. and dog, and rare in the horse, although more subject to the violent exertion which they put in the front of all causes. It is probable that the sluggish, pampered life of the first three animals, and the tendency to fatty degenerations and heart disease introduces a special predisposition as it does in man, while the horse, inured to an open air life and a vigorous muscular condition, is comparatively immune. Bright’s disease is a common cause in the human subject, with its resulting cardiac hypertrophy. The degenerations attendant on these conditions and especially fatty change (atheroma) in the walls of the cerebral arteries, pave the way for their rupture and for blood effusion. Emboli also carried from the diseased heart not only cut off the blood from the parts supplied by the plugged arteries, but increase the blood tension on the cardiac side of the obstruction and endanger rupture at any weak part. Thus they may cause apoplexy from anæmia without rupture or apoplexy from the pressure of effused blood.
Age which is such a notorious factor in man is not without its influence in the lower animals. It is in the old that we mostly see disease of kidneys and heart and the degenerations of the tissues, including the brain and its vessels; in these, therefore, rupture and extravasation are the most frequent.
The other causes are mostly connected with increased blood tension with or without a debility of the vascular walls. Violent exertions as in racing, coursing, dragging heavy loads up hill or on heavy ground, severe excitement, cerebral concussion, insolation, and intense congestion of the brain substance have all been recognized as causative factors. The compression of the jugulars by a small collar, the violent straining attendant on parturition, or constipation, and even the retrocession of blood from the surface when exposed to extreme cold, may contribute to the final rupture.
In infectious diseases in which the toxic products tend to produce profound modifications in the blood and tissues, extravasations are met with in the brain as in other organs. Thus they are seen in anthrax, Texas fever, petechial fever, etc.
Then the formation of neoplasms in the brain may be the occasion of the rupture of the vascular walls and apoplexy. Hæmatoma of the dura in the dog (Friedberger and Fröhner), cholesteatomata in the horse, and carcinoma may be apparent causes.
The effect of mechanical injury must be admitted, as blows on the head, injuries from an ox yoke, and concussions during the battles of rams and bulls.
Lesions. Blood extravasations may be found at any part of the brain: a. into the brain substance; b. into the ventricles; c. from the pia mater; d. into the arachnoid sac; e. between the skull and dura mater. It is especially common in connection with the ganglia adjoining the ventricles; the corpus striatum, optic thalamus, the corpora quadrigemini, the fornix. In other cases the crus cerebri, pons, medulla oblongata, corpus calosum. In other cases the convolutions of the cerebrum or cerebellum suffer. The amount of effusion may be limited to a few drops or it may cover an extensive area and cause considerable flattening of the brain substance.
When capillary hæmorrhages are present—the size of a millet seed or a pea—Friedberger and Fröhner have usually found them multiple, but when large enough to form distinct clots they are usually single and confined to one side. If a clot, involving the brain substance, is small, it merely separates the nervous fibres, but if larger, the cerebral tissue is broken down in the mass of clot, discolored, torn and softened. If the patient has survived the first attack the clot passes through the different stages of discoloration, brown, brownish yellow, yellow, and may become fibrous forming a distinct cicatrix, with loss of brain substance. In connection with the partial absorption of the effused blood, cavities may be filled with a serous fluid (apoplectic cysts), and these may show multiple loculi. The nerve fibres which lead to an old standing lesion are usually degenerated.
When effused into a ventricle, blood is less readily absorbed and tends to remain as a flattened discolored layer.
Extravasation between the dura mater and the cranium is probably always the result of direct mechanical violence.