The most frequent form of those found is the glioma. It is commonly found in the cerebrum, the cerebellum, the pons, and medulla. It is thought to never originate from the pia. The glioma appears as a diffuse, poorly defined area, pinkish or reddish in color from the numerous blood-vessels that are present. The tumor may be composed purely of neurogliar tissue, without any nerve-cells or fibers present. Occasionally ganglionic cells, isolated or in groups, are found embedded in the neoplasm; such tumors are known as gangliomata or neurogliomata. Microscopically gliomata are made up of cells from which numerous filaments project and which compose the groundworks of the tumor.

Some gliomata are considered as sarcoma, but as the tumors arise from different embryonal layers, such a combination could hardly occur. The sarcoma differs clinically from the ordinary glioma in being of a more rapid growth and giving metastases.

Sarcoma is fairly frequent, and commonly arises from the pia or from the connective tissue around the blood-vessels. It is generally rather circumscribed, and may be encapsulated. The commonest variety is the small round-cell, but spindle-cell and giant-cell forms occur. Angiosarcoma is not infrequent, and the perithelioma or myxangiosarcoma tubulare is fairly common. Psammosarcoma, in which infiltration of lime-salts takes place, is occasionally encountered. It is generally small and gritty. Endothelioma is found in the pia and sometimes in the choroid plexuses.

Other forms of primary tumors are rare. As secondary growths, sarcoma and carcinoma are fairly frequent.

Parasites are rare; the echinococcus and the cysticercus cellulosoe have been found.