774 



THE NERVOUS SYSTEM. 



serum, the attachments to the dura mater may disappear, and the whole 

 appearance becomes that of an independent cyst between the dura mater 

 and the pia mater. Fibroma, lipoma, myxoma, chondroma, and osteoma are 

 of rare occurrence. 



Endotheliomata. These tumors are of not infrequent occurrence, and 

 may grow from the pia of the cerebrum or cerebellum or from the chor- 

 oid plexus. They may be single or multiple. They may be small or so 

 large as seriously to compress the brain. 



Some of them are composed of a connective-tissue stroma which en- 

 closes regular spaces filled with large, flat, nucleated cells. These may 

 resemble carcinoma. Some of them are composed of a connective -tissue 





FIG. 509. ENDOTHELIOMA OF THE CEREBELLUM ORIGINATING IN THE PIA MATER. 

 Showing general appearance of the tumor (see Fig. 510) . 



stroma which forms cavities lined with cylindrical epithelium. In such 

 tumors the stroma may grow so as to form papillae covered with cylin- 

 drical epithelium ; or in addition there may be mucous degeneration of 

 the stroma. In some of them there is a connective-tissue stroma which 

 contains large numbers of blood-vessels. Around these blood-vessels are 

 arranged regular masses of polyhedral cells (Figs. 509 and 510). 



In some of them the stroma is scanty. The cells are numerous, large, 

 flat, and arranged in little globular masses or nests. If in these little 

 nests there is a deposition of the salts of lime, forming concretions like 

 the so-called "brain sand," the tumor is called a "psammoma." Some 

 of the tumors seem to be formed of very thin, nucleated membranes ar- 

 ranged concentrically like the layers of an onion. 



