THE NERVOUS SYSTEM. 775 



Some of the tumors are composed of balls or nests of large, flat cells, 

 with which are found crystals of cholesteriu "cholesteatonia." ' 



Sarcoma. Tumors belonging to the ordinary types of round- and fusi- 

 form-celled sarcoma, and of myxo-sarcoiua, are occasionally found in the 

 pia mater. 



Cysts. Small cysts are often found in the choroid plexus. Barely 

 such cysts reach a larger size, even as large as a pigeon's egg. 



Q ;..L- r ._ 



^. 





FIG. 510. EXDOTHELIOMA OF THE PlA MATER OF THE CEREBELLUM. 



From specimen shown in Fig. 417 more highly magnified. A, Section of pia mater dipping into a sulcus; 

 B, tumor cells growing at each side of the pia ; C, surface of cerebellar convolutions. 



( \ sts of the pia mater containing serum, with walls and septa of con- 

 nective tissue and compressing the brain, have been described. 



Variously shaped pigment cells not infrequently occur in the pia mater, 

 either scattered or sometimes in considerable masses ; they seem to have 

 little pathological significance. Xot infrequently thin plates of new- 

 formed bone are found in the pia mater, associated with a thickening of 

 the membrane. 



PARASITES. 

 Cysticercus has been observed in the pia mater. 



1 For a critical review of these and other pial tumors, with bibliographj', see Bost- 

 roem, Ceutrbl. f. allg. Path. u. path. Anat, Bd. viii., p. 1, 1897. 



