DISEASES OF THE BRAIN. 297 



severe symptoms until they have attained a size perhaps as 

 large as a pigeon's egg. They are sometimes met with as 

 large as a hen's egg. They occur in horses of various ages, 

 but are more common in old animals. The tumours are 

 composed of cholesterine* (CogH^^O), of calcareous particles 

 termed * brain sand,' and a little serum. They are enveloped 

 in the folds of the choroid plexus, encapsuled by a more or 

 less dense membrane, and held together in many instances 

 by fibrous tissue in variable amount. 



These tumours are developed in connection with hyper- 

 trophied villi of the choroid plexus, which are frequently 

 enlarged and congested in old horses. Sometimes they are 

 made up almost entirely of cholesterine arranged in spherical 

 masses, and are probably due to caseous and calcareous 

 degeneration of inflammatory exudation thrown out by the 

 processes of the plexus.1 



The calcareous particles are contained in concentric bodies 

 called corpora amylacea, and are termed ' brain sand.' 



These concentric bodies, held together by fibrous tissue, 

 are sometimes met with in the brain and its membranes 

 in man, and are termed ' psammomata ' (from -vj^a/x/xa, sand). 

 In some instances these calcareous bodies form a large part 

 of the tumours in horses, while in others cholesterine is 

 the chief constituent. 



The lateral ventricles may be almost, or quite, filled by 

 the tumours, and not uncommonly congestion and other signs 



■^ Cholesterine, a normal constituent of the bile and various tissues, 

 especially the nervous, is a neutral substance which readily crystallizes 

 from an ethereal solution in rhombic plates, which, when seen in mass, 

 have a mother-of-pearl lustre. It fuses at 145°, is insoluble in water 

 but soluble in alcohol, ether, and chloroform. When evaporated with 

 nitric acid, the residue on addition of ammonia acquires a dull red 

 colour. If sulphuric acid be added to an equal amount of solution of 

 cholesterine in ether, the solution becomes red, then purple, while the 

 subjacent layers acquire a distinct green fluorescence. Cholesterine is 

 the chief constituent of gall-stones, and occurs in several other patho- 

 logical products. 



