Occurrence. Etiology. 645 



dilatation of these cavities, an increase in the size of the brain, 

 and an elevation of the cranial pressure. The disease may be 

 primary or secondary to some other disease of the brain. 



Occurrence. The disease is seen most frequently in the 

 horse in which animal it is the commonest cause of sleepy stag- 

 gers. Up to the present there is no very exact information as 

 to its frequency. Having regard to the anatomical observations 

 made by Dexler, it remains to make investigations to decide 

 how often the condition is the cause of staggers, and how often 

 it is the result of some other diseased condition of the brain. 

 No stress can be laid on the results obtained in the past because 

 no comparison was made with regard to the anatomical altera- 

 tions found, and measurement of the size of the ventricles al- 

 lows of a great chance of error. 



There are some oases on record of dropsy of the ventricles in the dog (Froh- 

 ner saw 20 eases among 70,000 dogs), the pig (Schindelka), and also in other 

 species, l;ut either the true nature of the disease is not represented, or no proof 

 has been adduced that the cases are actually cases of rdironic internal hydrocephalus. 

 In many cases the possibility of confusion with other chronic brain lesions not 

 recognizable with the naked eye is not excluded. 



Etiology. The lateral ventricles of the brain communicate 

 with the third ventricle through the foramen of Monro, and this 

 is in communication with the fourth ventricle by means of the 

 aqueduct of Svlvius. As shown by Dexler the cerebrospinal 

 fluid mav reach the sulj-arachnoidal space from the fourth ven- 

 tricle by \vav of the two lateral foramina, and here be reabsorbed 

 by the Veins. Cerebrospinal fluid may collect in the ventricles 

 first if an abnormally large amount of serum escapes from the 

 venous plexuses in the ventricles and cannot pass freely through 

 the relatively small aqueduct, or when the outflow of the other- 

 wise normal amount of liquid is prevented owing to a narrow- 

 ing or obstruction of any of the paths of communication. Dex- 

 ler was the first to supply a satisfactory explanation of the 

 collection of the cerebrospinal fluid. According to this the imme- 

 diate cause of chronic internal hydrocephalus is stenosis or 

 occlusion of the aqueducts of the brain. 



Dexler 's investigations have shown that the tentorium cerebelli in the horse 

 is for the most part ossified and that its membranous part is composed of very 

 tough connective tissue. The tentorium encloses a space measuring 4.2 cm. m 

 height and 3.5 cm. in width, allowing for individual variations, and terminates 

 above in an angle. This space is occupie.l by a very small portion of the vermiform 

 process of the cereliellum and the corpora qua.lrigemina together with the aqueduct 

 of Sylvius and the crura cerebri. The oral surface of the tentorium cerebelli lies 

 behind the convex surfaces of both occipital lobes of the hemispheres; the medial sur- 

 faces of these coming into contact with each other. In view of the fact that the bram 

 is enclosed in a bony case and that the brain tissue which contains a large amount 

 of fluid is incompressible, pulsation of the brain at each systole is only possible by 

 the expansion of the brain at the systole into the subarachnoid space, the ventricles, 

 and into the cavity of the tentorium cerebelli. Wince the tentorir.ni is very rigid, 

 pulsation of the brain in the horse is conveyed only to a comparatively small 

 portion of the brain Iving behind the tentorium. Consequently the portions of the 

 occipital lobes resting on the walls of the tentorial cavity are pressed into the ope^ 



