Occurrence. Etiology. 763 



Occurrence. Attacks of vertigo are chiefly noticed in 

 horses and dogs. Generally it is, with very few exceptions, 

 that draught horses fall ill, whereas blooded horses and those 

 of the hardy country type are seldom afflicted. 



Etiology. In the domesticated animals megrims scarcely 

 ever occurs as an independent affection (vertigo idiopathica s. 

 essentialis), but as a rule as a secondary ailment (vertigo 

 symptomatica). 



Brain diseases may above all form the basis of megrims. 

 Hyperemia, hydrocephalus, tumors, parasites, contusion and 

 hemorrhage into the brain, emboli of the cerebral vessels, in- 

 flammation of the brain and its coverings come under this cate- 

 gory in about equal proportions. Also diseases of the cerebel- 

 lum and its adjoining parts of the brain are, in all probability, 

 often accompanied by true vertigo. 



Sometimes vertigo is associated with defects of vision. 

 Observations of attacks of megrims through disturbance of the 

 motor power of the eye muscles have not been recorded but it 

 is certain that suitable external irritation of the organs of 

 vision can produce attacks of staggers, as many observations 

 prove. Thus horses are at times attacked with vertigo when 

 travelling on unequally lighted highways, between two rows of 

 trees, or when moving quickly under a rising or setting sun, 

 when running round in a circle for a long time, or when objects 

 move quickl}^ before their eyes as in railway journeys. Besides 

 many authors blame the glistening inner surface of the blink- 

 ers which reflect rays of light for causing attacks of vertigo. 



Of diseases of the organ of hearing, affections of the laby- 

 rinth or of the n. vestibularis may occasion megrims (compare 

 Meniere's disease). 



Disturbances of circulation may result in vertigo (n. car- 

 diaca) such as every form of heart weakness, compression of 

 the veins, pericarditis, growths inside the pericardium or in 

 the neighl)orhood of the base of the heart. In this connection, 

 compression of the superficial veins in otherwise healthy horses 

 may be noted; this may be due to parts of the harness (the col- 

 lar or throat latch) or to the head being pulled and held in (in 

 draught horses). Far more readily may cerebral anemia cause 

 attacks of megrims. 



Of the diseases of the digestive organs, intestinal catarrh or 

 helminthiasis (v. verminosa) sometimes lead to attacks of dizzi- 

 ness. 



Violent irritation of the skin may occasion an attack es- 

 pecially if occurring on the injured skin; Guibert has seen an 

 attack of vertigo after simply brushing over a short clipped 

 fetlock. 



Finally megrims occurs as a symptom of acute poisoning 

 and indeed as a result of a direct eff'ect of the poison on the 

 central nervous system. Such an effect is produced by alcohol 



