This is a tumour with the structure of the connective tissue of the central nervous system, the Neuroglia. In examining a section of the brain substance, it is difficult to tell what is really nervous structure, and what the supporting connective substance, but when we examine the surfaces of the ventricles we find that the ganglion cells and nerve-fibres fall away, and just at the surface or ependyma we have what is presumably a purely connective substance. When hardened sections are examined, this is seen to consist of a finely reticulated network of fibres and round or slightly elongated cells. In the fresh state the fibres are not obvious, and we have a granular material. This connective substance has some of the characters of mucous tissue, and seems allied to it.
The glioma as it occurs in the brain does not usually form an isolated tumour, but, being continuous with the brain substance, has more the appearance of a swelling of part of the brain. It is seen also that the different shades of colour of different parts of the brain are lost when a glioma takes their place. Gliomas are usually soft in consistence and grey in colour, so as to resemble brain substance, but thev sometimes attain considerable density. They sometimes occur as small granular or warty projections on the surface of the ventricles, but the more important ones involve considerable portions of the brain substance.
Under the microscope the glioma is seen to resemble the neuroglia, but the cells are much more abundant. There is a well-developed fine or coarse network, and in it cells with oval nuclei. (See Fig. 89.) The cells present considerable variety in size.
Fig. 89. - Glioma of brain.
Gliomas being soft and somewhat cellular tumours, are liable to secondary changes. Haemorrhage not infrequently occurs, and the blood causing pressure around, the case may end like one of haemor-rhagic apoplexy. The tumour may also undergo fatty or caseous metamorphosis, and if a limited haemorrhage has occurred the clot may change in a similar way. In this manner a tumour which had originally the appearance of brain substance may change considerably.
The tumour is usually of slow growth and non-malignant, except in the sense that on account of its site it often affects important parts and causes death.
Gliomas occur also in the retina, forming soft tumours which fill up the eyeball. The true glioma is an innocent tumour, but sometimes it assumes a sarcomatous character and malignancy is developed.