Etiology. Anatomical Changes. 647 



Rarely the disease may be set up in other ways. The for- 

 amen of Monro may be occhided by parasites (echinococci, 

 ccenuri), tumors in the neighboring tissues, closure of the lateral 

 foramina owing to chronic meningitis. Possibly cases of this 

 sort occur in other animals. 



Cholesteatomata* which are found comparatively frequently in connection 

 with the choroid plexus in the horse probably are not causally connected with 

 hydrocephalus iuteruus because they are as often absent in animals so diseased and 

 present in otherwise healthy horses as in animals that are affected with dropsy 

 of the ventricles. In exceptional cases the ventricles even when cholesteatoniata 

 are present as large as a nut or larger do not contain more liquid than normal. 



Predisposition. It is generally supposed that a predis- 

 position is hereditary. As a matter of fact experience shows 

 that thoroughbreds and racing breeds are rarely affected with 

 sleepy staggers, whereas heavy breeds are often so affected. It 

 appears to be not impossible that in heavy breeds the anatomical 

 formation of the cranium is less favorable (small cranium with 

 relatively large tentorial opening), permitting greater protru- 

 sion of the occipital lobes or exerting a prejudicial influence on 

 the pulsation of the brain. In view of the fact that the cranial 

 formation is a hereditary feature, predisposition may also be 

 transmitted from generation to generation. The fact should 

 not be lost sight of that the heavy breeds do the heaviest work 

 and consecpientl}^ are more likely to suffer from repeated ele- 

 vations of intracranial pressure. There is no recorded evi- 

 dence of the hereditary nature of the disease. 



in support of the hereditary theory it is stated that staggers is of so 

 common occurrence in many district:: and particularly in mountainous districts 

 that horse-breeding is impossible (Trasbot). The frequent occurrence of the 

 disease may be connected with local conditions such as chronic poisoning with bad 

 food or it may be connected with repeated infections which set up chronic diseases 

 of the brain. 



As a rule it is mares and geldings that are used for work 

 that suffer most. Trasbot saw the disease principally among 

 stallions. 



Age plays some part in the production of the disease. The 

 majority of cases occur in horses from six to fourteen years 

 old. The disease seldom occurs before that, and in animals 

 under two years old it has not been observed. The disease has 

 been connected with the changing of the teeth, but this cannot 

 play any part in its production. 



The occurrence of hydrocephalus ex vacuo has not as yet been observed in 

 animals, because animals rarely reach such an age that senile atrophy of the brain 

 tissue would lead to dilatation of the ventricles. 



Anatomical Changes. The lesions produced by the pres- 

 ence of liquid under pressure in the ventricles depend upon the 

 severity and duration of the disease. These may in part dis- 

 appear when the pressure is relieved, but they may persist up 



*See footnote, page 656. 



